carbon-lang/toolchain/check/import_ref.cpp

// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include "toolchain/check/import_ref.h"

#include <optional>
#include <tuple>
#include <type_traits>
#include <utility>

#include "common/check.h"
#include "common/growing_range.h"
#include "toolchain/base/kind_switch.h"
#include "toolchain/base/shared_value_stores.h"
#include "toolchain/check/context.h"
#include "toolchain/check/eval.h"
#include "toolchain/check/generic.h"
#include "toolchain/check/import.h"
#include "toolchain/check/inst.h"
#include "toolchain/check/name_lookup.h"
#include "toolchain/check/type.h"
#include "toolchain/check/type_completion.h"
#include "toolchain/parse/node_ids.h"
#include "toolchain/sem_ir/constant.h"
#include "toolchain/sem_ir/file.h"
#include "toolchain/sem_ir/ids.h"
#include "toolchain/sem_ir/impl.h"
#include "toolchain/sem_ir/import_ir.h"
#include "toolchain/sem_ir/inst.h"
#include "toolchain/sem_ir/inst_categories.h"
#include "toolchain/sem_ir/inst_kind.h"
#include "toolchain/sem_ir/name_scope.h"
#include "toolchain/sem_ir/specific_interface.h"
#include "toolchain/sem_ir/specific_named_constraint.h"
#include "toolchain/sem_ir/type_info.h"
#include "toolchain/sem_ir/typed_insts.h"

namespace Carbon::Check {

// Adds the ImportIR, excluding the update to the check_ir_map.
static auto InternalAddImportIR(Context& context, SemIR::ImportIR import_ir)
    -> SemIR::ImportIRId {
  context.import_ir_constant_values().push_back(SemIR::ConstantValueStore(
      SemIR::ConstantId::None,
      import_ir.sem_ir ? &import_ir.sem_ir->insts() : nullptr));
  return context.import_irs().Add(import_ir);
}

// Adds a special-cased IR and verifies it received the correct ID.
static auto SetSpecialImportIR(Context& context, SemIR::ImportIR import_ir,
                               SemIR::ImportIRId expected_import_ir_id)
    -> void {
  auto ir_id = SemIR::ImportIRId::None;
  if (import_ir.sem_ir != nullptr) {
    ir_id = AddImportIR(context, import_ir);
  } else {
    // We don't have a check_ir_id, so add without touching check_ir_map.
    context.import_ir_constant_values().push_back(
        SemIR::ConstantValueStore(SemIR::ConstantValueStore::Unusable));
    ir_id = context.import_irs().Add(import_ir);
  }
  CARBON_CHECK(ir_id == expected_import_ir_id,
               "Actual ImportIRId ({0}) != Expected ImportIRId ({1})", ir_id,
               expected_import_ir_id);
}

auto SetSpecialImportIRs(Context& context, SemIR::ImportIR import_ir) -> void {
  SetSpecialImportIR(context, import_ir, SemIR::ImportIRId::ApiForImpl);
  SetSpecialImportIR(context,
                     {.decl_id = SemIR::InstId::None, .is_export = false},
                     SemIR::ImportIRId::Cpp);
}

auto AddImportIR(Context& context, SemIR::ImportIR import_ir)
    -> SemIR::ImportIRId {
  auto& ir_id = context.check_ir_map().Get(import_ir.sem_ir->check_ir_id());
  if (!ir_id.has_value()) {
    // Note this updates check_ir_map.
    ir_id = InternalAddImportIR(context, import_ir);
  } else if (import_ir.is_export) {
    // We're processing an `export import`. In case the IR was indirectly added
    // as a non-export, mark it as an export.
    context.import_irs().Get(ir_id).is_export = true;
  }
  return ir_id;
}

auto AddImportRef(Context& context, SemIR::ImportIRInst import_ir_inst,
                  SemIR::EntityNameId entity_name_id =
                      SemIR::EntityNameId::None) -> SemIR::InstId {
  auto import_ir_inst_id = context.import_ir_insts().Add(import_ir_inst);
  auto import_ref_id = AddPlaceholderImportedInstInNoBlock(
      context,
      MakeImportedLocIdAndInst(
          context, import_ir_inst_id,
          SemIR::ImportRefUnloaded{.import_ir_inst_id = import_ir_inst_id,
                                   .entity_name_id = entity_name_id}));
  return import_ref_id;
}

static auto GetCanonicalImportIRInst(Context& context,
                                     const SemIR::File* target_ir,
                                     SemIR::InstId target_inst_id)
    -> SemIR::ImportIRInst {
  auto [canonical_ir, canonical_inst_id] =
      GetCanonicalFileAndInstId(target_ir, target_inst_id);

  auto ir_id = SemIR::ImportIRId::None;
  if (canonical_ir != &context.sem_ir()) {
    // This uses AddImportIR in case it was indirectly found, which can
    // happen with two or more steps of exports.
    ir_id = AddImportIR(context, {.decl_id = SemIR::InstId::None,
                                  .is_export = false,
                                  .sem_ir = canonical_ir});
  }
  return SemIR::ImportIRInst(ir_id, canonical_inst_id);
}

auto GetCanonicalImportIRInst(Context& context, SemIR::InstId inst_id)
    -> SemIR::ImportIRInst {
  return GetCanonicalImportIRInst(context, &context.sem_ir(), inst_id);
}

auto VerifySameCanonicalImportIRInst(Context& context, SemIR::NameId name_id,
                                     SemIR::InstId prev_id,
                                     SemIR::ImportIRInst prev_import_ir_inst,
                                     SemIR::ImportIRId new_ir_id,
                                     const SemIR::File* new_import_ir,
                                     SemIR::InstId new_inst_id) -> void {
  auto new_import_ir_inst =
      GetCanonicalImportIRInst(context, new_import_ir, new_inst_id);
  if (new_import_ir_inst == prev_import_ir_inst) {
    return;
  }
  auto conflict_id =
      AddImportRef(context, SemIR::ImportIRInst(new_ir_id, new_inst_id));
  // TODO: Pass the imported name location instead of the conflict id.
  DiagnoseDuplicateName(context, name_id, SemIR::LocId(conflict_id),
                        SemIR::LocId(prev_id));
}

namespace {
// A context within which we are performing an import. Tracks information about
// the source and destination. This provides a restricted interface compared to
// ImportResolver: in particular, it does not have access to a work list.
// Therefore code that accepts an ImportContext is unable to enqueue new work.
class ImportContext {
 public:
  // A generic that we have partially imported.
  struct PendingGeneric {
    SemIR::GenericId import_id;
    SemIR::GenericId local_id;
  };

  // A specific that we have partially imported.
  struct PendingSpecific {
    SemIR::SpecificId import_id;
    SemIR::SpecificId local_id;
  };

  // `context` must not be null.
  explicit ImportContext(Context* context, SemIR::ImportIRId import_ir_id)
      : context_(context),
        import_ir_id_(import_ir_id),
        import_ir_(*context_->import_irs().Get(import_ir_id).sem_ir) {}

  // Returns the file we are importing from.
  auto import_ir() -> const SemIR::File& { return import_ir_; }

  // Accessors into value stores of the file we are importing from.
  auto import_associated_constants() -> const SemIR::AssociatedConstantStore& {
    return import_ir().associated_constants();
  }
  auto import_classes() -> const SemIR::ClassStore& {
    return import_ir().classes();
  }
  auto import_vtables() -> const SemIR::VtableStore& {
    return import_ir().vtables();
  }
  auto import_constant_values() -> const SemIR::ConstantValueStore& {
    return import_ir().constant_values();
  }
  auto import_entity_names() -> const SemIR::EntityNameStore& {
    return import_ir().entity_names();
  }
  auto import_facet_types() -> const SemIR::FacetTypeInfoStore& {
    return import_ir().facet_types();
  }
  auto import_functions() -> const SemIR::FunctionStore& {
    return import_ir().functions();
  }
  auto import_generics() -> const SemIR::GenericStore& {
    return import_ir().generics();
  }
  auto import_identifiers() -> const SharedValueStores::IdentifierStore& {
    return import_ir().identifiers();
  }
  auto import_impls() -> const SemIR::ImplStore& { return import_ir().impls(); }
  auto import_inst_blocks() -> const SemIR::InstBlockStore& {
    return import_ir().inst_blocks();
  }
  auto import_insts() -> const SemIR::InstStore& { return import_ir().insts(); }
  auto import_interfaces() -> const SemIR::InterfaceStore& {
    return import_ir().interfaces();
  }
  auto import_named_constraints() -> const SemIR::NamedConstraintStore& {
    return import_ir().named_constraints();
  }
  auto import_ints() -> const SharedValueStores::IntStore& {
    return import_ir().ints();
  }
  auto import_name_scopes() -> const SemIR::NameScopeStore& {
    return import_ir().name_scopes();
  }
  auto import_require_impls() -> const SemIR::RequireImplsStore& {
    return import_ir().require_impls();
  }
  auto import_require_impls_blocks() -> const SemIR::RequireImplsBlockStore& {
    return import_ir().require_impls_blocks();
  }
  auto import_specifics() -> const SemIR::SpecificStore& {
    return import_ir().specifics();
  }
  auto import_specific_interfaces() -> const SemIR::SpecificInterfaceStore& {
    return import_ir().specific_interfaces();
  }
  auto import_string_literal_values()
      -> const SharedValueStores::StringLiteralStore& {
    return import_ir().string_literal_values();
  }
  auto import_struct_type_fields() -> const SemIR::StructTypeFieldsStore& {
    return import_ir().struct_type_fields();
  }
  auto import_types() -> const SemIR::TypeStore& { return import_ir().types(); }

  // Returns the local file's import ID for the IR we are importing from.
  auto import_ir_id() -> SemIR::ImportIRId { return import_ir_id_; }

  // A value store for local constant values of imported instructions. This maps
  // from `InstId`s in the import IR to corresponding `ConstantId`s in the local
  // IR.
  auto local_constant_values_for_import_insts() -> SemIR::ConstantValueStore& {
    return local_context().import_ir_constant_values()
        [local_context().sem_ir().import_irs().GetRawIndex(import_ir_id_)];
  }

  // Returns the file we are importing into.
  auto local_ir() -> SemIR::File& { return context_->sem_ir(); }

  // Returns the type-checking context we are importing into.
  auto local_context() -> Context& { return *context_; }

  // Accessors into value stores of the file we are importing into.
  auto local_associated_constants() -> SemIR::AssociatedConstantStore& {
    return local_ir().associated_constants();
  }
  auto local_classes() -> SemIR::ClassStore& { return local_ir().classes(); }
  auto local_vtables() -> SemIR::VtableStore& { return local_ir().vtables(); }
  auto local_constant_values() -> SemIR::ConstantValueStore& {
    return local_ir().constant_values();
  }
  auto local_entity_names() -> SemIR::EntityNameStore& {
    return local_ir().entity_names();
  }
  auto local_facet_types() -> SemIR::FacetTypeInfoStore& {
    return local_ir().facet_types();
  }
  auto local_functions() -> SemIR::FunctionStore& {
    return local_ir().functions();
  }
  auto local_generics() -> SemIR::GenericStore& {
    return local_ir().generics();
  }
  auto local_identifiers() -> SharedValueStores::IdentifierStore& {
    return local_ir().identifiers();
  }
  auto local_impls() -> SemIR::ImplStore& { return local_ir().impls(); }
  auto local_import_ir_insts() -> SemIR::ImportIRInstStore& {
    return local_ir().import_ir_insts();
  }
  auto local_inst_blocks() -> SemIR::InstBlockStore& {
    return local_ir().inst_blocks();
  }
  auto local_insts() -> SemIR::InstStore& { return local_ir().insts(); }
  auto local_interfaces() -> SemIR::InterfaceStore& {
    return local_ir().interfaces();
  }
  auto local_named_constraints() -> SemIR::NamedConstraintStore& {
    return local_ir().named_constraints();
  }
  auto local_ints() -> SharedValueStores::IntStore& {
    return local_ir().ints();
  }
  auto local_name_scopes() -> SemIR::NameScopeStore& {
    return local_ir().name_scopes();
  }
  auto local_require_impls() -> SemIR::RequireImplsStore& {
    return local_ir().require_impls();
  }
  auto local_require_impls_blocks() -> SemIR::RequireImplsBlockStore& {
    return local_ir().require_impls_blocks();
  }
  auto local_specifics() -> SemIR::SpecificStore& {
    return local_ir().specifics();
  }
  auto local_specific_interfaces() -> SemIR::SpecificInterfaceStore& {
    return local_ir().specific_interfaces();
  }
  auto local_string_literal_values() -> SharedValueStores::StringLiteralStore& {
    return local_ir().string_literal_values();
  }
  auto local_struct_type_fields() -> SemIR::StructTypeFieldsStore& {
    return local_ir().struct_type_fields();
  }
  auto local_types() -> SemIR::TypeStore& { return local_ir().types(); }

  // Add a generic that has been partially imported but needs to be finished.
  auto AddPendingGeneric(PendingGeneric pending) -> void {
    pending_generics_.push_back(pending);
  }

  // Add a specific that has been partially imported but needs to be finished.
  auto AddPendingSpecific(PendingSpecific pending) -> void {
    pending_specifics_.push_back(pending);
  }

 protected:
  auto pending_generics() -> llvm::SmallVector<PendingGeneric>& {
    return pending_generics_;
  }
  auto pending_specifics() -> llvm::SmallVector<PendingSpecific>& {
    return pending_specifics_;
  }

 private:
  Context* context_;
  SemIR::ImportIRId import_ir_id_;
  const SemIR::File& import_ir_;

  // TODO: The following members don't belong here. This pending work mechanism
  // can probably be removed entirely if we stop importing generic eval blocks
  // and instead evaluate them directly in the imported IR.

  // Generics that we have partially imported but not yet finished importing.
  llvm::SmallVector<PendingGeneric> pending_generics_;
  // Specifics that we have partially imported but not yet finished importing.
  llvm::SmallVector<PendingSpecific> pending_specifics_;
};

// Resolves an instruction from an imported IR into a constant referring to the
// current IR.
//
// Calling Resolve on an instruction operates in an iterative manner, tracking
// Work items on work_stack_. At a high level, the loop is:
//
// 1. If a constant value is already known for the work item and was not set by
//    this work item, it's considered resolved.
//    - The constant check avoids performance costs of deduplication on add.
//    - If we've processed this work item before, then we now process it again.
//      It didn't complete last time, even though we have a constant value
//      already.
//
// 2. Resolve the instruction (TryResolveInst/TryResolveTypedInst). This is done
//    in three phases. The first and second phases can add work to the worklist
//    and end in a retry, in which case those phases will be rerun once the
//    added work is done. The rerun cannot also end in a retry, so this results
//    in at most three calls, but in practice one or two calls is almost always
//    sufficient. Due to the chance of a second or third call to TryResolveInst,
//    it's important to only perform expensive work once, even when the same
//    phase is rerun.
//
//    - First phase:
//      - Gather all input constants necessary to form the constant value of the
//        instruction. Gathering constants directly adds unresolved values to
//        work_stack_.
//      - If HasNewWork() reports that any work was added, then return Retry():
//        this instruction needs another call to complete. Gather the
//        now-resolved constants and continue to the next step once the retry
//        happens.
//
//    - Second phase:
//      - Build the constant value of the instruction.
//      - Gather all input constants necessary to finish importing the
//        instruction. This is only necessary for instructions like classes that
//        can be forward-declared. For these instructions, we first import the
//        constant value and then later import the rest of the declaration in
//        order to break cycles.
//      - If HasNewWork() reports that any work was added, then return
//        Retry(constant_value): this instruction needs another call to
//        complete.  Gather the now-resolved constants and continue to the next
//        step once the retry happens.
//
//    - Third phase:
//      - After the second phase, the constant value for the instruction is
//        already set, and will be passed back into TryResolve*Inst on retry. It
//        should not be created again.
//      - Fill in any remaining information to complete the import of the
//        instruction. For example, when importing a class declaration, build
//        the class scope and information about the definition.
//      - Return Done() to finish the resolution process. This will cause the
//        Resolve loop to set a constant value if we didn't retry at the end of
//        the second phase.
//
//    In the common case where the second phase cannot add new work (because the
//    inst doesn't represent a declaration of an entity that can be forward
//    declared), the second and third phases are usually expressed as a call to
//    ResolveResult::Deduplicated or ResolveResult::Unique.
//
// 3. If resolve didn't return Retry(), pop the work. Otherwise, it needs to
//    remain, and may no longer be at the top of the stack; update the state of
//    the work item to track what work still needs to be done.
//
// The same instruction can be enqueued for resolution multiple times. However,
// we will only reach the second phase once: once a constant value is set, only
// the resolution step that set it will retry.
//
// TODO: Fix class `extern` handling and merging, rewrite tests.
// - check/testdata/class/cross_package_import.carbon
// - check/testdata/class/extern.carbon
// TODO: Fix function `extern` handling and merging, rewrite tests.
// - check/testdata/function/declaration/import.carbon
// - check/testdata/packages/cross_package_import.carbon
class ImportRefResolver : public ImportContext {
 public:
  // `context` must not be null.
  explicit ImportRefResolver(Context* context, SemIR::ImportIRId import_ir_id)
      : ImportContext(context, import_ir_id) {}

  // Iteratively resolves an imported instruction's inner references until a
  // constant ID referencing the current IR is produced. See the class comment
  // for more details.
  auto ResolveOneInst(SemIR::InstId inst_id) -> SemIR::ConstantId;

  // Performs resolution for one instruction and then performs all work we
  // deferred.
  auto Resolve(SemIR::InstId inst_id) -> SemIR::ConstantId {
    auto const_id = ResolveOneInst(inst_id);
    PerformPendingWork();
    return const_id;
  }

  // Wraps constant evaluation with logic to handle constants.
  auto ResolveConstant(SemIR::ConstantId import_const_id) -> SemIR::ConstantId {
    return Resolve(GetInstWithConstantValue(import_ir(), import_const_id));
  }

  // Wraps constant evaluation with logic to handle types.
  auto ResolveType(SemIR::TypeId import_type_id) -> SemIR::TypeId;

  // Returns true if new unresolved constants were found as part of this
  // `Resolve` step.
  auto HasNewWork() -> bool {
    CARBON_CHECK(initial_work_ <= work_stack_.size(),
                 "Work shouldn't decrease");
    return initial_work_ < work_stack_.size();
  }

  // Returns the ConstantId for an InstId. Adds unresolved constants to
  // work_stack_.
  auto GetLocalConstantValueOrPush(SemIR::InstId inst_id) -> SemIR::ConstantId;

 private:
  // A step in work_stack_.
  struct Work {
    // The instruction to work on.
    SemIR::InstId inst_id;
    // Whether this work item set the constant value for the instruction and
    // requested a retry.
    bool retry_with_constant_value = false;
  };

  // The constant found by FindResolvedConstId.
  struct ResolvedConstId {
    // The constant for the instruction. `None` if not yet resolved.
    SemIR::ConstantId const_id = SemIR::ConstantId::None;

    // Instructions which are indirect but equivalent to the current instruction
    // being resolved, and should have their constant set to the same. Empty
    // when `const_id` has a value.
    llvm::SmallVector<SemIR::ImportIRInst> indirect_insts = {};
  };

  // Looks to see if an instruction has been resolved. If a constant is only
  // found indirectly, sets the constant for any indirect steps that don't
  // already have the constant. If a constant isn't found, returns the indirect
  // instructions so that they can have the resolved constant assigned later.
  auto FindResolvedConstId(SemIR::InstId inst_id) -> ResolvedConstId;

  // Sets a resolved constant into the current and indirect instructions.
  auto SetResolvedConstId(SemIR::InstId inst_id,
                          llvm::ArrayRef<SemIR::ImportIRInst> indirect_insts,
                          SemIR::ConstantId const_id) -> void;

  auto PerformPendingWork() -> void;

  llvm::SmallVector<Work> work_stack_;
  // The size of work_stack_ at the start of resolving the current instruction.
  size_t initial_work_ = 0;
};
}  // namespace

// Wrapper for `AddImportRef` that provides the `import_ir_id`.
static auto AddImportRef(ImportContext& context, SemIR::InstId inst_id,
                         SemIR::EntityNameId entity_name_id =
                             SemIR::EntityNameId::None) -> SemIR::InstId {
  return AddImportRef(context.local_context(),
                      SemIR::ImportIRInst(context.import_ir_id(), inst_id),
                      entity_name_id);
}

// Handles setting a constant on instructions related to an import.
static auto SetIndirectConstantValues(
    Context& context, llvm::ArrayRef<SemIR::ImportIRInst> indirect_insts,
    SemIR::ConstantId constant_id) -> void {
  for (const auto& import_ir_inst : indirect_insts) {
    auto ir_index =
        context.sem_ir().import_irs().GetRawIndex(import_ir_inst.ir_id());
    context.import_ir_constant_values()[ir_index].Set(import_ir_inst.inst_id(),
                                                      constant_id);
  }
}

// Adds an import_ref instruction for an instruction that we have already loaded
// from an imported IR, with a known constant value. This is useful when the
// instruction has a symbolic constant value, in order to produce an instruction
// that hold that symbolic constant.
static auto AddLoadedImportRef(ImportContext& context,
                               SemIR::TypeId local_type_id,
                               SemIR::InstId import_inst_id,
                               SemIR::ConstantId local_const_id)
    -> SemIR::InstId {
  auto import_ir_inst_id = context.local_import_ir_insts().Add(
      SemIR::ImportIRInst(context.import_ir_id(), import_inst_id));
  SemIR::ImportRefLoaded inst = {.type_id = local_type_id,
                                 .import_ir_inst_id = import_ir_inst_id,
                                 .entity_name_id = SemIR::EntityNameId::None};
  auto inst_id = AddPlaceholderImportedInstInNoBlock(
      context.local_context(),
      MakeImportedLocIdAndInst(context.local_context(), import_ir_inst_id,
                               inst));

  context.local_constant_values().Set(inst_id, local_const_id);
  context.local_constant_values_for_import_insts().Set(import_inst_id,
                                                       local_const_id);
  return inst_id;
}

// Like `AddLoadedImportRef`, but only for types, and returns a `TypeInstId`.
static auto AddLoadedImportRefForType(ImportContext& context,
                                      SemIR::TypeInstId import_inst_id,
                                      SemIR::ConstantId local_const_id)
    -> SemIR::TypeInstId {
  return context.local_types().GetAsTypeInstId(AddLoadedImportRef(
      context, SemIR::TypeType::TypeId, import_inst_id, local_const_id));
}

static auto AddImportIRInst(ImportContext& context, SemIR::InstId inst_id)
    -> SemIR::ImportIRInstId {
  return context.local_import_ir_insts().Add(
      SemIR::ImportIRInst(context.import_ir_id(), inst_id));
}

// Computes, sets, and returns the constant value for an instruction.
static auto SetConstantValue(Context& context, SemIR::InstId inst_id,
                             SemIR::Inst inst) -> SemIR::ConstantId {
  auto const_id = TryEvalInstUnsafe(context, inst_id, inst);
  if (const_id.is_constant()) {
    CARBON_VLOG_TO(context.vlog_stream(), "Constant: {0} -> {1}\n", inst,
                   context.constant_values().GetInstId(const_id));
  }
  context.constant_values().Set(inst_id, const_id);
  return const_id;
}

// Adds an imported instruction without setting its constant value. The
// instruction should later be updated by either `SetConstantValue` or
// `ReplacePlaceholderImportedInst`.
template <typename InstT>
static auto AddPlaceholderImportedInst(ImportContext& context,
                                       SemIR::InstId import_inst_id, InstT inst)
    -> SemIR::InstId {
  auto import_ir_inst_id = AddImportIRInst(context, import_inst_id);
  return AddPlaceholderImportedInstInNoBlock(
      context.local_context(),
      MakeImportedLocIdAndInst(context.local_context(), import_ir_inst_id,
                               inst));
}

// Replace an imported instruction that was added by
// `AddPlaceholderImportedInst` with a new instruction. Computes, sets, and
// returns the new constant value.
static auto ReplacePlaceholderImportedInst(ImportContext& context,
                                           SemIR::InstId inst_id,
                                           SemIR::Inst inst)
    -> SemIR::ConstantId {
  CARBON_VLOG_TO(context.local_context().vlog_stream(),
                 "ReplaceImportedInst: {0} -> {1}\n", inst_id, inst);
  context.local_insts().Set(inst_id, inst);

  CARBON_CHECK(context.local_constant_values().Get(inst_id) ==
               SemIR::ConstantId::None);
  return SetConstantValue(context.local_context(), inst_id, inst);
}

// Returns the ConstantId for an InstId. Adds unresolved constants to
// the resolver's work stack.
static auto GetLocalConstantId(ImportRefResolver& resolver,
                               SemIR::InstId inst_id) -> SemIR::ConstantId {
  return resolver.GetLocalConstantValueOrPush(inst_id);
}

// Returns the ConstantId for an imported ConstantId. Adds unresolved
// constants to the resolver's work stack.
static auto GetLocalConstantId(ImportRefResolver& resolver,
                               SemIR::ConstantId const_id)
    -> SemIR::ConstantId {
  return GetLocalConstantId(
      resolver, GetInstWithConstantValue(resolver.import_ir(), const_id));
}

// Returns the local constant InstId for an imported InstId.
static auto GetLocalConstantInstId(ImportRefResolver& resolver,
                                   SemIR::InstId inst_id) -> SemIR::InstId {
  auto const_id = GetLocalConstantId(resolver, inst_id);
  return resolver.local_constant_values().GetInstIdIfValid(const_id);
}

// Returns the local constant InstId for an imported InstId.
static auto GetLocalTypeInstId(ImportRefResolver& resolver,
                               SemIR::TypeInstId inst_id) -> SemIR::TypeInstId {
  // The input instruction is a TypeInstId, and import does not change the type
  // of instructions, so the result is also a valid TypeInstId.
  return SemIR::TypeInstId::UnsafeMake(
      GetLocalConstantInstId(resolver, inst_id));
}

// Returns the ConstantId for a TypeId. Adds unresolved constants to
// work_stack_.
static auto GetLocalConstantId(ImportRefResolver& resolver,
                               SemIR::TypeId type_id) -> SemIR::ConstantId {
  return GetLocalConstantId(resolver,
                            resolver.import_types().GetConstantId(type_id));
}

// Translates a NameId from the import IR to a local NameId.
//
// No new work is generated by calling this function.
static auto GetLocalNameId(ImportContext& context, SemIR::NameId import_name_id)
    -> SemIR::NameId {
  if (auto ident_id = import_name_id.AsIdentifierId(); ident_id.has_value()) {
    return SemIR::NameId::ForIdentifier(context.local_identifiers().Add(
        context.import_identifiers().Get(ident_id)));
  }
  return import_name_id;
}

// Returns the id for a local symbolic EntityName from an imported one,
// preserving only the `NameId`, the `CompileTimeBindIndex`, and whether it is a
// template. Other parts of the EntityName are not kept and are not considered
// part of the canonical EntityName (even if they are present there).
//
// No new work is generated by calling this function.
static auto GetLocalSymbolicEntityNameId(
    ImportContext& context, SemIR::EntityNameId import_entity_name_id)
    -> SemIR::EntityNameId {
  const auto& import_entity_name =
      context.import_entity_names().Get(import_entity_name_id);
  auto name_id = GetLocalNameId(context, import_entity_name.name_id);
  return context.local_entity_names().AddSymbolicBindingName(
      name_id, SemIR::NameScopeId::None, import_entity_name.bind_index(),
      import_entity_name.is_template);
}

// Gets the local constant values corresponding to an imported inst block.
static auto GetLocalInstBlockContents(ImportRefResolver& resolver,
                                      SemIR::InstBlockId import_block_id)
    -> llvm::SmallVector<SemIR::InstId> {
  llvm::SmallVector<SemIR::InstId> inst_ids;
  if (!import_block_id.has_value() ||
      import_block_id == SemIR::InstBlockId::Empty) {
    return inst_ids;
  }

  // Import all the values in the block.
  auto import_block = resolver.import_inst_blocks().Get(import_block_id);
  inst_ids.reserve(import_block.size());
  for (auto import_inst_id : import_block) {
    inst_ids.push_back(GetLocalConstantInstId(resolver, import_inst_id));
  }

  return inst_ids;
}

// Gets a local canonical instruction block ID corresponding to an imported inst
// block whose contents were already imported, for example by
// GetLocalInstBlockContents.
static auto GetLocalCanonicalInstBlockId(ImportContext& context,
                                         SemIR::InstBlockId import_block_id,
                                         llvm::ArrayRef<SemIR::InstId> contents)
    -> SemIR::InstBlockId {
  if (!import_block_id.has_value()) {
    return SemIR::InstBlockId::None;
  }
  return context.local_inst_blocks().AddCanonical(contents);
}

// Imports the RequireImplsDecl instructions for each RequireImplsId in the
// block, and gets the local RequireImplsIds from them. The returned vector is
// only complete if there is no more work to do in the resolver on return.
static auto GetLocalRequireImplsBlockContents(
    ImportRefResolver& resolver, SemIR::RequireImplsBlockId import_block_id)
    -> llvm::SmallVector<SemIR::RequireImplsId> {
  llvm::SmallVector<SemIR::RequireImplsId> require_decl_ids;
  if (!import_block_id.has_value() ||
      import_block_id == SemIR::RequireImplsBlockId::Empty) {
    return require_decl_ids;
  }

  // Import the RequireImplsDecl for each RequireImpls in the block.
  auto import_block =
      resolver.import_require_impls_blocks().Get(import_block_id);
  require_decl_ids.reserve(import_block.size());
  for (auto import_require_impls_id : import_block) {
    const auto& import_require =
        resolver.import_require_impls().Get(import_require_impls_id);
    auto local_decl_id =
        GetLocalConstantInstId(resolver, import_require.decl_id);
    // If `local_decl_id` is None, the resolver will have more work to do, and
    // we will call this function to try get all the decl instructions again.
    if (local_decl_id.has_value()) {
      // Importing the RequireImplsDecl instruction in `local_decl_id` also
      // imported the RequireImpls structure that it points to through the
      // RequireImplsId.
      require_decl_ids.push_back(
          resolver.local_insts()
              .GetAs<SemIR::RequireImplsDecl>(local_decl_id)
              .require_impls_id);
    }
  }

  return require_decl_ids;
}

// Gets the local block of RequireImplsIds from the imported block. Only valid
// to call once there is no more work to do after the call to
// GetLocalRequireImplsBlockContents().
static auto GetLocalCanonicalRequireImplsBlockId(
    ImportContext& context, SemIR::RequireImplsBlockId import_block_id,
    llvm::ArrayRef<SemIR::RequireImplsId> contents)
    -> SemIR::RequireImplsBlockId {
  if (!import_block_id.has_value()) {
    return SemIR::RequireImplsBlockId::None;
  }
  return context.local_require_impls_blocks().Add(contents);
}

// Gets a local instruction block containing ImportRefs referring to the
// instructions in the specified imported instruction block.
static auto GetLocalImportRefInstBlock(ImportContext& context,
                                       SemIR::InstBlockId import_inst_block_id)
    -> SemIR::InstBlockId {
  llvm::SmallVector<SemIR::InstId> elements;
  auto import_elements = context.import_inst_blocks().Get(import_inst_block_id);
  elements.reserve(import_elements.size());
  for (auto element : import_elements) {
    elements.push_back(AddImportRef(context, element));
  }
  return context.local_inst_blocks().Add(elements);
}

// Gets an incomplete local version of an imported generic. Most fields are
// set in the third phase.
static auto MakeIncompleteGeneric(ImportContext& context, SemIR::InstId decl_id,
                                  SemIR::GenericId generic_id)
    -> SemIR::GenericId {
  if (!generic_id.has_value()) {
    return SemIR::GenericId::None;
  }

  return context.local_generics().Add(
      {.decl_id = decl_id,
       .bindings_id = SemIR::InstBlockId::None,
       .self_specific_id = SemIR::SpecificId::None});
}

namespace {
// Local information associated with an imported generic.
struct GenericData {
  struct Binding {
    // The attached type's constant, which may differ from the type on the
    // constant. This needs to be preserved for the ImportRef.
    SemIR::ConstantId type_constant_id;
    SemIR::ConstantId inst_constant_id;
  };
  llvm::SmallVector<Binding> bindings;

  llvm::SmallVector<SemIR::InstId> decl_block;
};
}  // namespace

// Gets a local version of the data associated with a generic.
static auto GetLocalGenericData(ImportRefResolver& resolver,
                                SemIR::GenericId import_generic_id)
    -> GenericData {
  GenericData generic_data;
  if (import_generic_id.has_value()) {
    const auto& import_generic =
        resolver.import_generics().Get(import_generic_id);

    if (import_generic.bindings_id.has_value()) {
      auto import_bindings =
          resolver.import_inst_blocks().Get(import_generic.bindings_id);
      generic_data.bindings.reserve(import_bindings.size());
      for (auto import_inst_id : import_bindings) {
        generic_data.bindings.push_back(
            {.type_constant_id = GetLocalConstantId(
                 resolver,
                 resolver.import_insts().GetAttachedType(import_inst_id)),
             .inst_constant_id = GetLocalConstantId(resolver, import_inst_id)});
      }
    }

    generic_data.decl_block =
        GetLocalInstBlockContents(resolver, import_generic.decl_block_id);
  }
  return generic_data;
}

// Adds the given local generic data to the given generic.
static auto SetGenericData(ImportContext& context,
                           SemIR::GenericId import_generic_id,
                           SemIR::GenericId new_generic_id,
                           const GenericData& generic_data) -> void {
  if (!import_generic_id.has_value()) {
    return;
  }

  const auto& import_generic = context.import_generics().Get(import_generic_id);
  auto& new_generic = context.local_generics().Get(new_generic_id);

  auto import_bindings =
      context.import_inst_blocks().Get(import_generic.bindings_id);
  llvm::SmallVector<SemIR::InstId> new_bindings;
  new_bindings.reserve(import_bindings.size());
  for (auto [import_binding_id, binding] :
       llvm::zip_equal(import_bindings, generic_data.bindings)) {
    auto local_type_id = context.local_types().GetTypeIdForTypeConstantId(
        binding.type_constant_id);
    new_bindings.push_back(AddLoadedImportRef(
        context, local_type_id, import_binding_id, binding.inst_constant_id));
  }
  new_generic.bindings_id = context.local_inst_blocks().Add(new_bindings);

  // TODO: Import the generic eval block rather than calling
  // RebuildGenericEvalBlock to rebuild it so that order doesn't matter.
  new_generic.decl_block_id = RebuildGenericEvalBlock(
      context.local_context(), new_generic_id,
      SemIR::GenericInstIndex::Region::Declaration, generic_data.decl_block);

  // Track that we need to fill in the remaining information in
  // FinishPendingGeneric.
  context.AddPendingGeneric(
      {.import_id = import_generic_id, .local_id = new_generic_id});
}

// Gets a local constant value corresponding to an imported generic ID. May
// add work to the work stack and return `None`.
static auto GetLocalConstantId(ImportRefResolver& resolver,
                               SemIR::GenericId generic_id)
    -> SemIR::ConstantId {
  if (!generic_id.has_value()) {
    return SemIR::ConstantId::None;
  }
  auto import_decl_inst_id = resolver.import_generics().Get(generic_id).decl_id;
  auto import_decl_inst =
      resolver.import_insts().GetWithAttachedType(import_decl_inst_id);
  if (import_decl_inst.Is<SemIR::ImplDecl>()) {
    // For an impl declaration, the imported entity can be found via the
    // declaration.
    return GetLocalConstantId(resolver, import_decl_inst_id);
  }
  // For all other kinds of declaration, the imported entity can be found via
  // the type of the declaration.
  return GetLocalConstantId(resolver, import_decl_inst.type_id());
}

// Gets a local generic ID given the corresponding local constant ID returned
// by GetLocalConstantId for the imported generic. Does not add any new work.
static auto GetLocalGenericId(ImportContext& context,
                              SemIR::ConstantId local_const_id)
    -> SemIR::GenericId {
  if (!local_const_id.has_value()) {
    return SemIR::GenericId::None;
  }
  auto inst = context.local_insts().Get(
      context.local_constant_values().GetInstId(local_const_id));
  CARBON_KIND_SWITCH(inst) {
    case CARBON_KIND(SemIR::FunctionType fn_type): {
      return context.local_functions().Get(fn_type.function_id).generic_id;
    }
    case CARBON_KIND(SemIR::GenericClassType class_type): {
      return context.local_classes().Get(class_type.class_id).generic_id;
    }
    case CARBON_KIND(SemIR::GenericInterfaceType interface_type): {
      return context.local_interfaces()
          .Get(interface_type.interface_id)
          .generic_id;
    }
    case CARBON_KIND(SemIR::GenericNamedConstraintType constraint_type): {
      return context.local_named_constraints()
          .Get(constraint_type.named_constraint_id)
          .generic_id;
    }
    case CARBON_KIND(SemIR::ImplDecl impl_decl): {
      return context.local_impls().Get(impl_decl.impl_id).generic_id;
    }
    default: {
      CARBON_FATAL("Unexpected inst for generic declaration: {0}", inst);
    }
  }
}

namespace {
// Local information associated with an imported specific.
struct SpecificData {
  SemIR::ConstantId generic_const_id;
  llvm::SmallVector<SemIR::InstId> args;
};
}  // namespace

// Gets local information about an imported specific.
static auto GetLocalSpecificData(ImportRefResolver& resolver,
                                 SemIR::SpecificId specific_id)
    -> SpecificData {
  if (!specific_id.has_value()) {
    return {.generic_const_id = SemIR::ConstantId::None, .args = {}};
  }

  const auto& specific = resolver.import_specifics().Get(specific_id);
  return {
      .generic_const_id = GetLocalConstantId(resolver, specific.generic_id),
      .args = GetLocalInstBlockContents(resolver, specific.args_id),
  };
}

// Gets a local specific whose data was already imported by
// GetLocalSpecificData. Does not add any new work.
static auto GetOrAddLocalSpecific(ImportContext& context,
                                  SemIR::SpecificId import_specific_id,
                                  const SpecificData& data)
    -> SemIR::SpecificId {
  if (!import_specific_id.has_value()) {
    return SemIR::SpecificId::None;
  }

  // Form a corresponding local specific ID.
  const auto& import_specific =
      context.import_specifics().Get(import_specific_id);
  auto generic_id = GetLocalGenericId(context, data.generic_const_id);
  auto args_id =
      GetLocalCanonicalInstBlockId(context, import_specific.args_id, data.args);

  // Get the specific.
  auto specific_id = context.local_specifics().GetOrAdd(generic_id, args_id);

  // Fill in the remaining information in FinishPendingSpecific, if necessary.
  auto& specific = context.local_specifics().Get(specific_id);
  if (!specific.decl_block_id.has_value() ||
      (import_specific.definition_block_id.has_value() &&
       !specific.definition_block_id.has_value())) {
    context.AddPendingSpecific(
        {.import_id = import_specific_id, .local_id = specific_id});
  }
  return specific_id;
}

namespace {
struct SpecificInterfaceData {
  SemIR::ConstantId interface_const_id;
  SpecificData specific_data;
};
}  // namespace

static auto GetLocalSpecificInterfaceData(
    ImportRefResolver& resolver, SemIR::SpecificInterface import_interface)
    -> SpecificInterfaceData {
  SemIR::ConstantId interface_const_id = SemIR::ConstantId::None;
  if (import_interface.interface_id.has_value()) {
    interface_const_id =
        GetLocalConstantId(resolver, resolver.import_interfaces()
                                         .Get(import_interface.interface_id)
                                         .first_owning_decl_id);
  }
  return {.interface_const_id = interface_const_id,
          .specific_data =
              GetLocalSpecificData(resolver, import_interface.specific_id)};
}

static auto GetLocalSpecificInterface(
    ImportContext& context, SemIR::SpecificInterface import_specific_interface,
    SpecificInterfaceData interface_data) -> SemIR::SpecificInterface {
  if (!interface_data.interface_const_id.has_value()) {
    return SemIR::SpecificInterface::None;
  }
  // Find the corresponding interface type. For a non-generic interface,
  // this is the type of the interface declaration. For a generic interface,
  // build a interface type referencing this specialization of the generic
  // interface.
  auto interface_const_inst =
      context.local_insts().Get(context.local_constant_values().GetInstId(
          interface_data.interface_const_id));
  if (auto facet_type = interface_const_inst.TryAs<SemIR::FacetType>()) {
    const SemIR::FacetTypeInfo& new_facet_type_info =
        context.local_facet_types().Get(facet_type->facet_type_id);
    return std::get<SemIR::SpecificInterface>(
        *new_facet_type_info.TryAsSingleExtend());
  } else {
    auto generic_interface_type =
        context.local_types().GetAs<SemIR::GenericInterfaceType>(
            interface_const_inst.type_id());
    auto specific_id =
        GetOrAddLocalSpecific(context, import_specific_interface.specific_id,
                              interface_data.specific_data);
    return {generic_interface_type.interface_id, specific_id};
  }
}

namespace {
struct SpecificNamedConstraintData {
  SemIR::ConstantId constraint_const_id;
  SpecificData specific_data;
};
}  // namespace

static auto GetLocalSpecificNamedConstraintData(
    ImportRefResolver& resolver,
    SemIR::SpecificNamedConstraint import_constraint)
    -> SpecificNamedConstraintData {
  SemIR::ConstantId constraint_const_id = SemIR::ConstantId::None;
  if (import_constraint.named_constraint_id.has_value()) {
    constraint_const_id = GetLocalConstantId(
        resolver, resolver.import_named_constraints()
                      .Get(import_constraint.named_constraint_id)
                      .first_owning_decl_id);
  }
  return {.constraint_const_id = constraint_const_id,
          .specific_data =
              GetLocalSpecificData(resolver, import_constraint.specific_id)};
}

static auto GetLocalSpecificNamedConstraint(
    ImportContext& context,
    SemIR::SpecificNamedConstraint import_specific_constraint,
    SpecificNamedConstraintData constraint_data)
    -> SemIR::SpecificNamedConstraint {
  if (!constraint_data.constraint_const_id.has_value()) {
    return SemIR::SpecificNamedConstraint::None;
  }
  // Find the corresponding named constraint type. For a non-generic constraint,
  // this is the type of the named constraint declaration. For a generic
  // constraint, build a named constraint type referencing this specialization
  // of the generic named constraint.
  auto constraint_const_inst =
      context.local_insts().Get(context.local_constant_values().GetInstId(
          constraint_data.constraint_const_id));
  if (auto facet_type = constraint_const_inst.TryAs<SemIR::FacetType>()) {
    const SemIR::FacetTypeInfo& new_facet_type_info =
        context.local_facet_types().Get(facet_type->facet_type_id);
    return std::get<SemIR::SpecificNamedConstraint>(
        *new_facet_type_info.TryAsSingleExtend());
  } else {
    auto generic_constraint_type =
        context.local_types().GetAs<SemIR::GenericNamedConstraintType>(
            constraint_const_inst.type_id());
    auto specific_id =
        GetOrAddLocalSpecific(context, import_specific_constraint.specific_id,
                              constraint_data.specific_data);
    return {generic_constraint_type.named_constraint_id, specific_id};
  }
}

static auto GetLocalNameScopeIdImpl(ImportRefResolver& resolver,
                                    SemIR::ConstantId const_id)
    -> SemIR::NameScopeId {
  if (!const_id.has_value()) {
    return SemIR::NameScopeId::None;
  }

  auto const_inst_id = resolver.local_constant_values().GetInstId(const_id);
  auto name_scope_inst = resolver.local_insts().Get(const_inst_id);
  CARBON_KIND_SWITCH(name_scope_inst) {
    case CARBON_KIND(SemIR::Namespace inst): {
      return inst.name_scope_id;
    }
    case CARBON_KIND(SemIR::ClassType inst): {
      return resolver.local_classes().Get(inst.class_id).scope_id;
    }
    case CARBON_KIND(SemIR::FacetType inst): {
      const SemIR::FacetTypeInfo& facet_type_info =
          resolver.local_facet_types().Get(inst.facet_type_id);
      if (auto single = facet_type_info.TryAsSingleExtend()) {
        // This is the facet type produced by an interface or named constraint
        // declaration.
        CARBON_KIND_SWITCH(*single) {
          case CARBON_KIND(SemIR::SpecificInterface interface): {
            return resolver.local_interfaces()
                .Get(interface.interface_id)
                .scope_id;
          }
          case CARBON_KIND(SemIR::SpecificNamedConstraint constraint): {
            return resolver.local_named_constraints()
                .Get(constraint.named_constraint_id)
                .scope_id;
          }
        }
      }
      break;
    }
    case CARBON_KIND(SemIR::ImplDecl inst): {
      return resolver.local_impls().Get(inst.impl_id).scope_id;
    }
    case SemIR::StructValue::Kind: {
      auto type_inst =
          resolver.local_types().GetAsInst(name_scope_inst.type_id());
      CARBON_KIND_SWITCH(type_inst) {
        case CARBON_KIND(SemIR::GenericClassType inst): {
          return resolver.local_classes().Get(inst.class_id).scope_id;
        }
        case CARBON_KIND(SemIR::GenericInterfaceType inst): {
          return resolver.local_interfaces().Get(inst.interface_id).scope_id;
        }
        case CARBON_KIND(SemIR::GenericNamedConstraintType inst): {
          return resolver.local_named_constraints()
              .Get(inst.named_constraint_id)
              .scope_id;
        }
        default: {
          break;
        }
      }
      break;
    }
    default: {
      if (const_inst_id == SemIR::ErrorInst::InstId) {
        return SemIR::NameScopeId::None;
      }
      break;
    }
  }
  CARBON_FATAL("Unexpected instruction kind for name scope: {0}",
               name_scope_inst);
}

// Translates a NameScopeId from the import IR to a local NameScopeId. Adds
// unresolved constants to the resolver's work stack.
static auto GetLocalNameScopeId(ImportRefResolver& resolver,
                                SemIR::NameScopeId name_scope_id)
    -> SemIR::NameScopeId {
  // Get the instruction that created the scope.
  auto [inst_id, inst] =
      resolver.import_name_scopes().GetInstIfValid(name_scope_id);
  if (!inst) {
    // Map scopes that aren't associated with an instruction to `None`. For now,
    // such scopes aren't used, and we don't have a good way to remap them.
    return SemIR::NameScopeId::None;
  }

  // Get the constant value for the scope.
  auto const_id = GetLocalConstantId(resolver, inst_id);
  if (!const_id.has_value()) {
    return SemIR::NameScopeId::None;
  }
  auto result = GetLocalNameScopeIdImpl(resolver, const_id);
  CARBON_CHECK(result.has_value());
  return result;
}

// Given an imported entity base, returns an incomplete, local version of it.
//
// Most fields are set in the third phase once they're imported. Import enough
// of the parameter lists that we know whether this interface is a generic
// interface and can build the right constant value for it.
//
// TODO: Support extern.
static auto GetIncompleteLocalEntityBase(
    ImportContext& context, SemIR::InstId decl_id,
    const SemIR::EntityWithParamsBase& import_base)
    -> SemIR::EntityWithParamsBase {
  // Translate the extern_library_id if present.
  auto extern_library_id = SemIR::LibraryNameId::None;
  if (import_base.extern_library_id.has_value()) {
    if (import_base.extern_library_id.index >= 0) {
      auto val = context.import_string_literal_values().Get(
          import_base.extern_library_id.AsStringLiteralValueId());
      extern_library_id = SemIR::LibraryNameId::ForStringLiteralValueId(
          context.local_string_literal_values().Add(val));
    } else {
      extern_library_id = import_base.extern_library_id;
    }
  }

  return {
      .name_id = GetLocalNameId(context, import_base.name_id),
      .parent_scope_id = SemIR::NameScopeId::None,
      .generic_id =
          MakeIncompleteGeneric(context, decl_id, import_base.generic_id),
      .first_param_node_id = Parse::NodeId::None,
      .last_param_node_id = Parse::NodeId::None,
      .pattern_block_id = SemIR::InstBlockId::None,
      .implicit_param_patterns_id =
          import_base.implicit_param_patterns_id.has_value()
              ? SemIR::InstBlockId::Empty
              : SemIR::InstBlockId::None,
      .param_patterns_id = import_base.param_patterns_id.has_value()
                               ? SemIR::InstBlockId::Empty
                               : SemIR::InstBlockId::None,
      .is_extern = import_base.is_extern,
      .extern_library_id = extern_library_id,
      .non_owning_decl_id = import_base.non_owning_decl_id.has_value()
                                ? decl_id
                                : SemIR::InstId::None,
      .first_owning_decl_id = import_base.first_owning_decl_id.has_value()
                                  ? decl_id
                                  : SemIR::InstId::None,
  };
}

// Adds ImportRefUnloaded entries for members of the imported scope, for name
// lookup.
static auto AddNameScopeImportRefs(ImportContext& context,
                                   const SemIR::NameScope& import_scope,
                                   SemIR::NameScope& new_scope) -> void {
  for (auto entry : import_scope.entries()) {
    SemIR::ScopeLookupResult result = entry.result;
    if (result.is_poisoned()) {
      continue;
    }
    auto ref_id = AddImportRef(context, result.target_inst_id());
    new_scope.AddRequired({.name_id = GetLocalNameId(context, entry.name_id),
                           .result = SemIR::ScopeLookupResult::MakeFound(
                               ref_id, result.access_kind())});
  }
  for (auto scope_inst_id : import_scope.extended_scopes()) {
    new_scope.AddExtendedScope(AddImportRef(context, scope_inst_id));
  }
}

// Given a block ID for a list of associated entities of a witness, returns a
// version localized to the current IR.
static auto AddAssociatedEntities(ImportContext& context,
                                  SemIR::NameScopeId local_name_scope_id,
                                  SemIR::InstBlockId associated_entities_id)
    -> SemIR::InstBlockId {
  if (associated_entities_id == SemIR::InstBlockId::Empty) {
    return SemIR::InstBlockId::Empty;
  }
  auto associated_entities =
      context.import_inst_blocks().Get(associated_entities_id);
  llvm::SmallVector<SemIR::InstId> new_associated_entities;
  new_associated_entities.reserve(associated_entities.size());
  for (auto inst_id : associated_entities) {
    // Determine the name of the associated entity, by switching on its kind.
    SemIR::NameId import_name_id = SemIR::NameId::None;
    if (auto assoc_const_decl =
            context.import_insts().TryGetAs<SemIR::AssociatedConstantDecl>(
                inst_id)) {
      const auto& assoc_const = context.import_associated_constants().Get(
          assoc_const_decl->assoc_const_id);
      import_name_id = assoc_const.name_id;
    } else if (auto function_decl =
                   context.import_insts().TryGetAs<SemIR::FunctionDecl>(
                       inst_id)) {
      const auto& function =
          context.import_functions().Get(function_decl->function_id);
      import_name_id = function.name_id;
    } else if (auto import_ref =
                   context.import_insts().TryGetAs<SemIR::AnyImportRef>(
                       inst_id)) {
      import_name_id =
          context.import_entity_names().Get(import_ref->entity_name_id).name_id;
    } else {
      // We don't need `GetWithAttachedType` here because we don't access the
      // type.
      CARBON_FATAL("Unhandled associated entity kind: {0}",
                   context.import_insts().Get(inst_id).kind());
    }
    auto name_id = GetLocalNameId(context, import_name_id);
    auto entity_name_id = context.local_entity_names().Add(
        {.name_id = name_id, .parent_scope_id = local_name_scope_id});
    new_associated_entities.push_back(
        AddImportRef(context, inst_id, entity_name_id));
  }
  return context.local_inst_blocks().Add(new_associated_entities);
}

namespace {
namespace Internal {
// Internal concept for instruction kinds that produce unique constants.
template <typename InstT>
concept HasUniqueConstantKind =
    InstT::Kind.constant_kind() == SemIR::InstConstantKind::AlwaysUnique ||
    InstT::Kind.constant_kind() == SemIR::InstConstantKind::ConditionalUnique;
}  // namespace Internal

// The result of attempting to resolve an imported instruction to a constant.
struct ResolveResult {
  // The new constant value, if known.
  SemIR::ConstantId const_id;
  // Newly created declaration whose value is being resolved, if any.
  SemIR::InstId decl_id = SemIR::InstId::None;
  // Whether resolution has been attempted once and needs to be retried.
  bool retry = false;

  // Produces a resolve result that tries resolving this instruction again. If
  // `const_id` is specified, then this is the end of the second phase, and the
  // constant value will be passed to the next resolution attempt. Otherwise,
  // this is the end of the first phase.
  static auto Retry(SemIR::ConstantId const_id = SemIR::ConstantId::None,
                    SemIR::InstId decl_id = SemIR::InstId::None)
      -> ResolveResult {
    return {.const_id = const_id, .decl_id = decl_id, .retry = true};
  }

  // Produces a resolve result that provides the given constant value. Requires
  // that there is no new work.
  static auto Done(SemIR::ConstantId const_id,
                   SemIR::InstId decl_id = SemIR::InstId::None)
      -> ResolveResult {
    return {.const_id = const_id, .decl_id = decl_id};
  }

  // Produces a resolve result that provides the given constant value. Retries
  // instead if work has been added.
  static auto RetryOrDone(ImportRefResolver& resolver,
                          SemIR::ConstantId const_id) -> ResolveResult {
    if (resolver.HasNewWork()) {
      return Retry();
    }
    return Done(const_id);
  }

  // Adds `inst` to the local context as a deduplicated constant and returns a
  // successful `ResolveResult`. Requires that there is no new work.
  //
  // This implements phases 2 and 3 of resolving the inst (as described on
  // `ImportRefResolver`) for the common case where those phases are combined.
  // Cases where that isn't applicable should instead use
  // `AddPlaceholderImportedInst` and `ReplacePlaceholderImportedInst`.
  //
  // This should not be used for instructions that represent declarations, or
  // other instructions with `constant_kind == InstConstantKind::Unique`,
  // because they should not be deduplicated.
  template <typename InstT>
    requires(!Internal::HasUniqueConstantKind<InstT>)
  static auto Deduplicated(ImportRefResolver& resolver, InstT inst)
      -> ResolveResult {
    CARBON_CHECK(!resolver.HasNewWork());
    // AddImportedConstant produces an unattached constant, so its type must
    // be unattached as well.
    inst.type_id =
        resolver.local_context().types().GetUnattachedType(inst.type_id);
    auto const_id = AddImportedConstant(resolver.local_context(), inst);
    CARBON_CHECK(const_id.is_constant(), "{0} is not constant", inst);
    return Done(const_id);
  }

  // Adds `inst` to the local context as a unique constant and returns a
  // successful `ResolveResult`. `import_inst_id` is the corresponding inst ID
  // in the local context. Requires that there is no new work.
  //
  // This implements phases 2 and 3 of resolving the inst (as described on
  // `ImportRefResolver`) for the common case where those phases are combined.
  // Cases where that isn't applicable should instead use
  // `AddPlaceholderImportedInst` and `ReplacePlaceholderImportedInst`.
  //
  // This should only be used for instructions that represent declarations, or
  // other instructions with `constant_kind == InstConstantKind::Unique`,
  // because it does not perform deduplication.
  template <typename InstT>
    requires Internal::HasUniqueConstantKind<InstT>
  static auto Unique(ImportRefResolver& resolver, SemIR::InstId import_inst_id,
                     InstT inst) -> ResolveResult {
    CARBON_CHECK(!resolver.HasNewWork());
    auto inst_id = AddPlaceholderImportedInst(resolver, import_inst_id, inst);
    auto const_id = SetConstantValue(resolver.local_context(), inst_id, inst);
    CARBON_CHECK(const_id.is_constant(), "{0} is not constant", inst);
    return Done(const_id, inst_id);
  }
};
}  // namespace

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::AdaptDecl inst,
                                SemIR::InstId import_inst_id) -> ResolveResult {
  auto adapted_type_const_id =
      GetLocalConstantId(resolver, inst.adapted_type_inst_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  auto adapted_type_inst_id = AddLoadedImportRefForType(
      resolver, inst.adapted_type_inst_id, adapted_type_const_id);

  // Create a corresponding instruction to represent the declaration.
  return ResolveResult::Unique<SemIR::AdaptDecl>(
      resolver, import_inst_id, {.adapted_type_inst_id = adapted_type_inst_id});
}

template <typename ParamPatternT>
  requires SemIR::Internal::HasInstCategory<SemIR::AnyParamPattern,
                                            ParamPatternT>
static auto TryResolveTypedInst(ImportRefResolver& resolver, ParamPatternT inst,
                                SemIR::InstId import_inst_id) -> ResolveResult {
  auto type_const_id = GetLocalConstantId(resolver, inst.type_id);
  auto subpattern_id = GetLocalConstantInstId(resolver, inst.subpattern_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Unique<ParamPatternT>(
      resolver, import_inst_id,
      {.type_id = resolver.local_context().types().GetTypeIdForTypeConstantId(
           type_const_id),
       .subpattern_id = subpattern_id,
       .index = inst.index});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::ArrayType inst) -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto element_type_inst_id =
      GetLocalTypeInstId(resolver, inst.element_type_inst_id);
  auto bound_id = GetLocalConstantInstId(resolver, inst.bound_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::ArrayType>(
      resolver, {.type_id = SemIR::TypeType::TypeId,
                 .bound_id = bound_id,
                 .element_type_inst_id = element_type_inst_id});
}

static auto MakeAssociatedConstant(
    ImportContext& context, const SemIR::AssociatedConstant& import_assoc_const,
    SemIR::TypeId type_id)
    -> std::pair<SemIR::AssociatedConstantId, SemIR::ConstantId> {
  SemIR::AssociatedConstantDecl assoc_const_decl = {
      .type_id = type_id,
      .assoc_const_id = SemIR::AssociatedConstantId::None,
      .decl_block_id = SemIR::InstBlockId::Empty};
  auto assoc_const_decl_id = AddPlaceholderImportedInst(
      context, import_assoc_const.decl_id, assoc_const_decl);
  assoc_const_decl.assoc_const_id = context.local_associated_constants().Add({
      .name_id = GetLocalNameId(context, import_assoc_const.name_id),
      .parent_scope_id = SemIR::NameScopeId::None,
      .decl_id = assoc_const_decl_id,
      .generic_id = MakeIncompleteGeneric(context, assoc_const_decl_id,
                                          import_assoc_const.generic_id),
      .default_value_id =
          import_assoc_const.default_value_id.has_value()
              ? AddImportRef(context, import_assoc_const.default_value_id)
              : SemIR::InstId::None,
  });

  // Write the associated constant ID into the AssociatedConstantDecl.
  auto const_id = ReplacePlaceholderImportedInst(context, assoc_const_decl_id,
                                                 assoc_const_decl);
  return {assoc_const_decl.assoc_const_id, const_id};
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::AssociatedConstantDecl inst,
                                SemIR::ConstantId const_id) -> ResolveResult {
  const auto& import_assoc_const =
      resolver.import_associated_constants().Get(inst.assoc_const_id);

  SemIR::AssociatedConstantId assoc_const_id =
      SemIR::AssociatedConstantId::None;
  if (!const_id.has_value()) {
    // In the first phase, import the type of the associated constant.
    auto type_const_id = GetLocalConstantId(resolver, inst.type_id);
    if (resolver.HasNewWork()) {
      return ResolveResult::Retry();
    }

    // In the second phase, create the associated constant and its declaration.
    auto type_id = resolver.local_context().types().GetTypeIdForTypeConstantId(
        type_const_id);
    std::tie(assoc_const_id, const_id) =
        MakeAssociatedConstant(resolver, import_assoc_const, type_id);
  } else {
    // In the third phase, compute the associated constant ID from the constant
    // value of the declaration.
    assoc_const_id =
        resolver.local_insts()
            .GetAs<SemIR::AssociatedConstantDecl>(
                resolver.local_constant_values().GetInstId(const_id))
            .assoc_const_id;
  }

  // Load the values to populate the entity with.
  auto parent_scope_id =
      GetLocalNameScopeId(resolver, import_assoc_const.parent_scope_id);
  auto generic_data =
      GetLocalGenericData(resolver, import_assoc_const.generic_id);
  auto& new_assoc_const =
      resolver.local_associated_constants().Get(assoc_const_id);

  if (resolver.HasNewWork()) {
    return ResolveResult::Retry(const_id, new_assoc_const.decl_id);
  }

  // Populate the entity.
  new_assoc_const.parent_scope_id = parent_scope_id;
  SetGenericData(resolver, import_assoc_const.generic_id,
                 new_assoc_const.generic_id, generic_data);
  return ResolveResult::Done(const_id, new_assoc_const.decl_id);
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::AssociatedEntity inst) -> ResolveResult {
  auto type_const_id = GetLocalConstantId(resolver, inst.type_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  // Add a lazy reference to the target declaration.
  auto decl_id = AddImportRef(resolver, inst.decl_id);

  return ResolveResult::Deduplicated<SemIR::AssociatedEntity>(
      resolver,
      {.type_id = resolver.local_context().types().GetTypeIdForTypeConstantId(
           type_const_id),
       .index = inst.index,
       .decl_id = decl_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::AssociatedEntityType inst)
    -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto data =
      GetLocalSpecificInterfaceData(resolver, inst.GetSpecificInterface());

  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  auto specific_interface =
      GetLocalSpecificInterface(resolver, inst.GetSpecificInterface(), data);
  return ResolveResult::Deduplicated<SemIR::AssociatedEntityType>(
      resolver, {.type_id = SemIR::TypeType::TypeId,
                 .interface_id = specific_interface.interface_id,
                 .interface_specific_id = specific_interface.specific_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::BaseDecl inst,
                                SemIR::InstId import_inst_id) -> ResolveResult {
  auto type_const_id = GetLocalConstantId(resolver, inst.type_id);
  auto base_type_const_id =
      GetLocalConstantId(resolver, inst.base_type_inst_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  auto base_type_inst_id = AddLoadedImportRefForType(
      resolver, inst.base_type_inst_id, base_type_const_id);

  // Create a corresponding instruction to represent the declaration.
  return ResolveResult::Unique<SemIR::BaseDecl>(
      resolver, import_inst_id,
      {.type_id = resolver.local_context().types().GetTypeIdForTypeConstantId(
           type_const_id),
       .base_type_inst_id = base_type_inst_id,
       .index = inst.index});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::AliasBinding inst) -> ResolveResult {
  auto value_id = GetLocalConstantId(resolver, inst.value_id);
  return ResolveResult::RetryOrDone(resolver, value_id);
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::SymbolicBinding inst) -> ResolveResult {
  auto type_id = GetLocalConstantId(resolver, inst.type_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  auto entity_name_id =
      GetLocalSymbolicEntityNameId(resolver, inst.entity_name_id);
  return ResolveResult::Deduplicated<SemIR::SymbolicBinding>(
      resolver,
      {.type_id =
           resolver.local_context().types().GetTypeIdForTypeConstantId(type_id),
       .entity_name_id = entity_name_id,
       .value_id = SemIR::InstId::None});
}

template <typename BindingPatternT>
  requires SemIR::Internal::HasInstCategory<SemIR::AnyBindingPattern,
                                            BindingPatternT>
static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                BindingPatternT inst,
                                SemIR::InstId import_inst_id) -> ResolveResult {
  auto type_const_id = GetLocalConstantId(resolver, inst.type_id);
  const auto& import_entity_name =
      resolver.import_entity_names().Get(inst.entity_name_id);
  auto parent_scope_id =
      GetLocalNameScopeId(resolver, import_entity_name.parent_scope_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  auto name_id = GetLocalNameId(resolver, import_entity_name.name_id);
  auto entity_name_id = resolver.local_entity_names().Add(
      {.name_id = name_id,
       .parent_scope_id = parent_scope_id,
       .bind_index_value = import_entity_name.bind_index().index,
       .is_template = import_entity_name.is_template});
  return ResolveResult::Unique<BindingPatternT>(
      resolver, import_inst_id,
      {.type_id = resolver.local_context().types().GetTypeIdForTypeConstantId(
           type_const_id),
       .entity_name_id = entity_name_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::BoolLiteral inst) -> ResolveResult {
  CARBON_CHECK(resolver.import_types().GetInstId(inst.type_id) ==
               SemIR::BoolType::TypeInstId);

  CARBON_CHECK(!resolver.HasNewWork());

  return ResolveResult::Deduplicated<SemIR::BoolLiteral>(
      resolver, {.type_id = GetSingletonType(resolver.local_context(),
                                             SemIR::BoolType::TypeInstId),
                 .value = inst.value});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::BoundMethod inst) -> ResolveResult {
  CARBON_CHECK(resolver.import_types().GetInstId(inst.type_id) ==
               SemIR::BoundMethodType::TypeInstId);
  auto object_id = GetLocalConstantInstId(resolver, inst.object_id);
  auto function_decl_id =
      GetLocalConstantInstId(resolver, inst.function_decl_id);

  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::BoundMethod>(
      resolver,
      {.type_id = GetSingletonType(resolver.local_context(),
                                   SemIR::BoundMethodType::TypeInstId),
       .object_id = object_id,
       .function_decl_id = function_decl_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver, SemIR::Call inst)
    -> ResolveResult {
  auto type_id = GetLocalConstantId(resolver, inst.type_id);
  auto callee_id = GetLocalConstantInstId(resolver, inst.callee_id);
  auto args = GetLocalInstBlockContents(resolver, inst.args_id);

  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::Call>(
      resolver,
      {.type_id =
           resolver.local_context().types().GetTypeIdForTypeConstantId(type_id),
       .callee_id = callee_id,
       .args_id = GetLocalCanonicalInstBlockId(resolver, inst.args_id, args)});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::CharLiteralValue inst) -> ResolveResult {
  CARBON_CHECK(resolver.import_types().GetInstId(inst.type_id) ==
               SemIR::CharLiteralType::TypeInstId);

  CARBON_CHECK(!resolver.HasNewWork());

  return ResolveResult::Deduplicated<SemIR::CharLiteralValue>(
      resolver,
      {.type_id = GetSingletonType(resolver.local_context(),
                                   SemIR::CharLiteralType::TypeInstId),
       .value = inst.value});
}

static auto AddPlaceholderNameScope(ImportContext& context)
    -> SemIR::NameScopeId {
  return context.local_name_scopes().Add(
      SemIR::InstId::None, SemIR::NameId::None, SemIR::NameScopeId::None);
}

// Makes an incomplete class. This is necessary even with classes with a
// complete declaration, because things such as `Self` may refer back to the
// type.
static auto MakeIncompleteClass(ImportContext& context,
                                const SemIR::Class& import_class,
                                SemIR::SpecificId enclosing_specific_id)
    -> std::pair<SemIR::ClassId, SemIR::ConstantId> {
  SemIR::ClassDecl class_decl = {.type_id = SemIR::TypeType::TypeId,
                                 .class_id = SemIR::ClassId::None,
                                 .decl_block_id = SemIR::InstBlockId::Empty};
  auto class_decl_id = AddPlaceholderImportedInst(
      context, import_class.latest_decl_id(), class_decl);
  // Regardless of whether ClassDecl is a complete type, we first need an
  // incomplete type so that any references have something to point at.
  class_decl.class_id = context.local_classes().Add(
      {GetIncompleteLocalEntityBase(context, class_decl_id, import_class),
       {.self_type_id = SemIR::TypeId::None,
        .inheritance_kind = import_class.inheritance_kind,
        .is_dynamic = import_class.is_dynamic,
        .scope_id = import_class.is_complete()
                        ? AddPlaceholderNameScope(context)
                        : SemIR::NameScopeId::None}});

  if (import_class.has_parameters()) {
    class_decl.type_id = GetGenericClassType(
        context.local_context(), class_decl.class_id, enclosing_specific_id);
  }

  // Write the class ID into the ClassDecl.
  auto self_const_id =
      ReplacePlaceholderImportedInst(context, class_decl_id, class_decl);
  return {class_decl.class_id, self_const_id};
}

static auto InitializeNameScopeAndImportRefs(
    ImportContext& context, const SemIR::NameScope& import_scope,
    SemIR::NameScope& new_scope, SemIR::InstId decl_id, SemIR::NameId name_id,
    SemIR::NameScopeId parent_scope_id) {
  new_scope.Set(decl_id, name_id, parent_scope_id);
  AddNameScopeImportRefs(context, import_scope, new_scope);
}

// Fills out the class definition for an incomplete class.
static auto AddClassDefinition(ImportContext& context,
                               const SemIR::Class& import_class,
                               SemIR::Class& new_class,
                               SemIR::InstId complete_type_witness_id,
                               SemIR::InstId base_id, SemIR::InstId adapt_id,
                               SemIR::InstId vtable_decl_id) -> void {
  new_class.definition_id = new_class.first_owning_decl_id;

  new_class.complete_type_witness_id = complete_type_witness_id;

  auto& new_scope = context.local_name_scopes().Get(new_class.scope_id);
  const auto& import_scope =
      context.import_name_scopes().Get(import_class.scope_id);

  // Push a block so that we can add scoped instructions to it.
  context.local_context().inst_block_stack().Push();
  InitializeNameScopeAndImportRefs(
      context, import_scope, new_scope, new_class.first_owning_decl_id,
      SemIR::NameId::None, new_class.parent_scope_id);
  new_class.body_block_id = context.local_context().inst_block_stack().Pop();

  if (import_class.base_id.has_value()) {
    new_class.base_id = base_id;
  }
  if (import_class.adapt_id.has_value()) {
    new_class.adapt_id = adapt_id;
  }
  if (import_class.vtable_decl_id.has_value()) {
    new_class.vtable_decl_id = vtable_decl_id;
  }
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::ClassDecl inst,
                                SemIR::ConstantId class_const_id)
    -> ResolveResult {
  // TODO: The handling of interfaces repeats a lot with the handling of
  // classes, and will likely also be repeated for named constraints and
  // choice types. Factor out some of this functionality.
  const auto& import_class = resolver.import_classes().Get(inst.class_id);

  SemIR::ClassId class_id = SemIR::ClassId::None;
  if (!class_const_id.has_value()) {
    auto import_specific_id = SemIR::SpecificId::None;
    if (auto import_generic_class_type =
            resolver.import_types().TryGetAs<SemIR::GenericClassType>(
                inst.type_id)) {
      import_specific_id = import_generic_class_type->enclosing_specific_id;
    }
    auto specific_data = GetLocalSpecificData(resolver, import_specific_id);
    if (resolver.HasNewWork()) {
      // This is the end of the first phase. Don't make a new class yet if
      // we already have new work.
      return ResolveResult::Retry();
    }

    // On the second phase, create a forward declaration of the class for any
    // recursive references.
    auto enclosing_specific_id =
        GetOrAddLocalSpecific(resolver, import_specific_id, specific_data);
    std::tie(class_id, class_const_id) =
        MakeIncompleteClass(resolver, import_class, enclosing_specific_id);
  } else {
    // On the third phase, compute the class ID from the constant
    // value of the declaration.
    auto class_const_inst = resolver.local_insts().Get(
        resolver.local_constant_values().GetInstId(class_const_id));
    if (auto class_type = class_const_inst.TryAs<SemIR::ClassType>()) {
      class_id = class_type->class_id;
    } else {
      auto generic_class_type =
          resolver.local_types().GetAs<SemIR::GenericClassType>(
              class_const_inst.type_id());
      class_id = generic_class_type.class_id;
    }
  }

  // Load constants for the definition.
  auto parent_scope_id =
      GetLocalNameScopeId(resolver, import_class.parent_scope_id);
  auto implicit_param_patterns = GetLocalInstBlockContents(
      resolver, import_class.implicit_param_patterns_id);
  auto param_patterns =
      GetLocalInstBlockContents(resolver, import_class.param_patterns_id);
  auto generic_data = GetLocalGenericData(resolver, import_class.generic_id);
  auto self_const_id = GetLocalConstantId(resolver, import_class.self_type_id);
  auto complete_type_witness_const_id =
      import_class.complete_type_witness_id.has_value()
          ? GetLocalConstantId(resolver, import_class.complete_type_witness_id)
          : SemIR::ConstantId::None;
  auto base_id = import_class.base_id.has_value()
                     ? GetLocalConstantInstId(resolver, import_class.base_id)
                     : SemIR::InstId::None;
  auto adapt_id = import_class.adapt_id.has_value()
                      ? GetLocalConstantInstId(resolver, import_class.adapt_id)
                      : SemIR::InstId::None;
  auto vtable_decl_id =
      import_class.vtable_decl_id.has_value()
          ? AddImportRef(resolver, import_class.vtable_decl_id)
          : SemIR::InstId::None;
  auto& new_class = resolver.local_classes().Get(class_id);

  if (resolver.HasNewWork()) {
    return ResolveResult::Retry(class_const_id, new_class.first_decl_id());
  }

  new_class.parent_scope_id = parent_scope_id;
  new_class.implicit_param_patterns_id = GetLocalCanonicalInstBlockId(
      resolver, import_class.implicit_param_patterns_id,
      implicit_param_patterns);
  new_class.param_patterns_id = GetLocalCanonicalInstBlockId(
      resolver, import_class.param_patterns_id, param_patterns);
  SetGenericData(resolver, import_class.generic_id, new_class.generic_id,
                 generic_data);
  new_class.self_type_id =
      resolver.local_context().types().GetTypeIdForTypeConstantId(
          self_const_id);

  if (import_class.is_complete()) {
    auto complete_type_witness_id = AddLoadedImportRef(
        resolver,
        GetSingletonType(resolver.local_context(),
                         SemIR::WitnessType::TypeInstId),
        import_class.complete_type_witness_id, complete_type_witness_const_id);
    AddClassDefinition(resolver, import_class, new_class,
                       complete_type_witness_id, base_id, adapt_id,
                       vtable_decl_id);
  }

  return ResolveResult::Done(class_const_id, new_class.first_decl_id());
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::ClassType inst) -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto class_const_id = GetLocalConstantId(
      resolver,
      resolver.import_classes().Get(inst.class_id).first_owning_decl_id);
  if (class_const_id == SemIR::ErrorInst::ConstantId) {
    // TODO: It should be possible to remove this once C++ imports work.
    return ResolveResult::Done(class_const_id);
  }

  auto specific_data = GetLocalSpecificData(resolver, inst.specific_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  // Find the corresponding class type. For a non-generic class, this is the
  // type of the class declaration. For a generic class, build a class type
  // referencing this specialization of the generic class.
  auto class_const_inst = resolver.local_insts().Get(
      resolver.local_constant_values().GetInstId(class_const_id));
  if (class_const_inst.Is<SemIR::ClassType>()) {
    return ResolveResult::Done(class_const_id);
  } else {
    auto generic_class_type =
        resolver.local_types().GetAs<SemIR::GenericClassType>(
            class_const_inst.type_id());
    auto specific_id =
        GetOrAddLocalSpecific(resolver, inst.specific_id, specific_data);
    return ResolveResult::Deduplicated<SemIR::ClassType>(
        resolver, {.type_id = SemIR::TypeType::TypeId,
                   .class_id = generic_class_type.class_id,
                   .specific_id = specific_id});
  }
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::CompleteTypeWitness inst)
    -> ResolveResult {
  CARBON_CHECK(resolver.import_types().GetInstId(inst.type_id) ==
               SemIR::WitnessType::TypeInstId);
  auto object_repr_type_inst_id =
      GetLocalTypeInstId(resolver, inst.object_repr_type_inst_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }
  return ResolveResult::Deduplicated<SemIR::CompleteTypeWitness>(
      resolver, {.type_id = GetSingletonType(resolver.local_context(),
                                             SemIR::WitnessType::TypeInstId),
                 .object_repr_type_inst_id = object_repr_type_inst_id});
}

template <typename InstT>
  requires SemIR::Internal::HasInstCategory<SemIR::AnyQualifiedType, InstT>
static auto TryResolveTypedInst(ImportRefResolver& resolver, InstT inst)
    -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto inner_id = GetLocalTypeInstId(resolver, inst.inner_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }
  return ResolveResult::Deduplicated<InstT>(
      resolver, {.type_id = SemIR::TypeType::TypeId, .inner_id = inner_id});
}

static auto HandleUnsupportedCppOverloadSet(ImportRefResolver& resolver,
                                            SemIR::CppOverloadSetId id) {
  // Supporting C++ overload resolution of imported functions is a large task,
  // which might require serializing and deserializing AST for using decl ids,
  // using modules and/or linking ASTs.
  resolver.local_context().TODO(
      SemIR::LocId::None,
      llvm::formatv(
          "Unsupported: Importing C++ function `{0}` indirectly",
          resolver.import_ir().names().GetAsStringIfIdentifier(
              resolver.import_ir().cpp_overload_sets().Get(id).name_id)));
  return ResolveResult::Done(SemIR::ErrorInst::ConstantId,
                             SemIR::ErrorInst::InstId);
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::CppOverloadSetType inst)
    -> ResolveResult {
  return HandleUnsupportedCppOverloadSet(resolver, inst.overload_set_id);
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::CppOverloadSetValue inst)
    -> ResolveResult {
  return HandleUnsupportedCppOverloadSet(resolver, inst.overload_set_id);
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::ExportDecl inst) -> ResolveResult {
  auto value_id = GetLocalConstantId(resolver, inst.value_id);
  return ResolveResult::RetryOrDone(resolver, value_id);
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::FieldDecl inst,
                                SemIR::InstId import_inst_id) -> ResolveResult {
  auto const_id = GetLocalConstantId(resolver, inst.type_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }
  return ResolveResult::Unique<SemIR::FieldDecl>(
      resolver, import_inst_id,
      {.type_id = resolver.local_context().types().GetTypeIdForTypeConstantId(
           const_id),
       .name_id = GetLocalNameId(resolver, inst.name_id),
       .index = inst.index});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::FloatLiteralValue inst)
    -> ResolveResult {
  CARBON_CHECK(resolver.import_types().GetInstId(inst.type_id) ==
               SemIR::FloatLiteralType::TypeInstId);

  CARBON_CHECK(!resolver.HasNewWork());

  auto real_id = resolver.local_ir().reals().Add(
      resolver.import_ir().reals().Get(inst.real_id));

  return ResolveResult::Deduplicated<SemIR::FloatLiteralValue>(
      resolver,
      {.type_id = GetSingletonType(resolver.local_context(),
                                   SemIR::FloatLiteralType::TypeInstId),
       .real_id = real_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::FloatType inst) -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto bit_width_id = GetLocalConstantInstId(resolver, inst.bit_width_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::FloatType>(
      resolver, {.type_id = SemIR::TypeType::TypeId,
                 .bit_width_id = bit_width_id,
                 .float_kind = inst.float_kind});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::FloatValue inst) -> ResolveResult {
  auto type_id = GetLocalConstantId(resolver, inst.type_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  auto float_id = resolver.local_ir().floats().Add(
      resolver.import_ir().floats().Get(inst.float_id));

  return ResolveResult::Deduplicated<SemIR::FloatValue>(
      resolver,
      {.type_id =
           resolver.local_context().types().GetTypeIdForTypeConstantId(type_id),
       .float_id = float_id});
}

// Make a declaration of a function. This is done as a separate step from
// importing the function declaration in order to resolve cycles.
static auto MakeFunctionDecl(ImportContext& context,
                             const SemIR::Function& import_function,
                             SemIR::SpecificId specific_id)
    -> std::pair<SemIR::FunctionId, SemIR::ConstantId> {
  SemIR::FunctionDecl function_decl = {
      .type_id = SemIR::TypeId::None,
      .function_id = SemIR::FunctionId::None,
      .decl_block_id = SemIR::InstBlockId::Empty};
  auto function_decl_id = AddPlaceholderImportedInst(
      context, import_function.first_decl_id(), function_decl);

  // Start with an incomplete function.
  function_decl.function_id = context.local_functions().Add(
      {GetIncompleteLocalEntityBase(context, function_decl_id, import_function),
       {.call_params_id = SemIR::InstBlockId::None,
        .return_slot_pattern_id = SemIR::InstId::None,
        .virtual_modifier = import_function.virtual_modifier,
        .virtual_index = import_function.virtual_index}});

  // Directly add the function type constant. Don't use `GetFunctionType`
  // because that will evaluate the function type, which we can't do if the
  // specific's value block is still pending.
  auto type_const_id = AddImportedConstant(
      context.local_context(),
      SemIR::FunctionType{.type_id = SemIR::TypeType::TypeId,
                          .function_id = function_decl.function_id,
                          .specific_id = specific_id});
  function_decl.type_id =
      context.local_types().GetTypeIdForTypeConstantId(type_const_id);

  // Write the function ID and type into the FunctionDecl.
  auto function_const_id =
      ReplacePlaceholderImportedInst(context, function_decl_id, function_decl);
  return {function_decl.function_id, function_const_id};
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::FunctionDecl inst,
                                SemIR::ConstantId function_const_id)
    -> ResolveResult {
  const auto& import_function =
      resolver.import_functions().Get(inst.function_id);

  SemIR::FunctionId function_id = SemIR::FunctionId::None;
  if (!function_const_id.has_value()) {
    auto import_specific_id = resolver.import_types()
                                  .GetAs<SemIR::FunctionType>(inst.type_id)
                                  .specific_id;
    auto specific_data = GetLocalSpecificData(resolver, import_specific_id);
    if (resolver.HasNewWork()) {
      // This is the end of the first phase. Don't make a new function yet if
      // we already have new work.
      return ResolveResult::Retry();
    }

    // On the second phase, create a forward declaration of the interface.
    auto specific_id =
        GetOrAddLocalSpecific(resolver, import_specific_id, specific_data);
    std::tie(function_id, function_const_id) =
        MakeFunctionDecl(resolver, import_function, specific_id);
  } else {
    // On the third phase, compute the function ID from the constant value of
    // the declaration.
    auto function_const_inst = resolver.local_insts().Get(
        resolver.local_constant_values().GetInstId(function_const_id));
    auto function_type = resolver.local_types().GetAs<SemIR::FunctionType>(
        function_const_inst.type_id());
    function_id = function_type.function_id;
  }

  auto return_type_const_id = SemIR::ConstantId::None;
  if (import_function.return_slot_pattern_id.has_value()) {
    return_type_const_id = GetLocalConstantId(
        resolver, resolver.import_insts().GetAttachedType(
                      import_function.return_slot_pattern_id));
  }
  auto parent_scope_id =
      GetLocalNameScopeId(resolver, import_function.parent_scope_id);
  auto implicit_param_patterns = GetLocalInstBlockContents(
      resolver, import_function.implicit_param_patterns_id);
  auto param_patterns =
      GetLocalInstBlockContents(resolver, import_function.param_patterns_id);
  auto generic_data = GetLocalGenericData(resolver, import_function.generic_id);
  auto self_param_id =
      GetLocalConstantInstId(resolver, import_function.self_param_id);
  auto return_slot_pattern_id =
      GetLocalConstantInstId(resolver, import_function.return_slot_pattern_id);

  auto& new_function = resolver.local_functions().Get(function_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry(function_const_id,
                                new_function.first_decl_id());
  }

  // Add the function declaration.
  new_function.parent_scope_id = parent_scope_id;
  new_function.implicit_param_patterns_id = GetLocalCanonicalInstBlockId(
      resolver, import_function.implicit_param_patterns_id,
      implicit_param_patterns);
  new_function.self_param_id = self_param_id;
  new_function.param_patterns_id = GetLocalCanonicalInstBlockId(
      resolver, import_function.param_patterns_id, param_patterns);
  new_function.return_slot_pattern_id = return_slot_pattern_id;
  SetGenericData(resolver, import_function.generic_id, new_function.generic_id,
                 generic_data);

  if (import_function.definition_id.has_value()) {
    new_function.definition_id = new_function.first_owning_decl_id;
  }

  switch (import_function.special_function_kind) {
    case SemIR::Function::SpecialFunctionKind::None: {
      break;
    }
    case SemIR::Function::SpecialFunctionKind::Builtin: {
      new_function.SetBuiltinFunction(import_function.builtin_function_kind());
      break;
    }
    case SemIR::Function::SpecialFunctionKind::Thunk: {
      auto entity_name_id = resolver.local_entity_names().AddCanonical(
          {.name_id = new_function.name_id,
           .parent_scope_id = new_function.parent_scope_id});
      new_function.SetThunk(AddImportRef(
          resolver, import_function.thunk_decl_id(), entity_name_id));
      break;
    }
    case SemIR::Function::SpecialFunctionKind::HasCppThunk: {
      resolver.local_context().TODO(SemIR::LocId::None,
                                    "Unsupported: Importing C++ functions that "
                                    "require thunks indirectly");
    }
  }

  return ResolveResult::Done(function_const_id, new_function.first_decl_id());
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::VtableDecl inst) -> ResolveResult {
  const auto& import_vtable = resolver.import_vtables().Get(inst.vtable_id);
  auto class_const_id =
      GetLocalConstantId(resolver, resolver.import_classes()
                                       .Get(import_vtable.class_id)
                                       .first_owning_decl_id);
  auto class_const_inst = resolver.local_insts().Get(
      resolver.local_constant_values().GetInstId(class_const_id));

  // TODO: Ensure the vtable is only imported once, in eg: if there's distinct
  // vtable constants (imported from multiple libraries using the vtable) that
  // refer to the same vtable, the vtable should still be singular.
  auto virtual_functions =
      resolver.import_inst_blocks().Get(import_vtable.virtual_functions_id);

  llvm::SmallVector<SemIR::InstId> lazy_virtual_functions;
  lazy_virtual_functions.reserve(virtual_functions.size());
  for (auto vtable_entry_id : virtual_functions) {
    auto local_attached_constant_id =
        GetLocalConstantId(resolver, vtable_entry_id);
    lazy_virtual_functions.push_back(
        resolver.local_constant_values().GetInstIdIfValid(
            local_attached_constant_id));
  }

  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  for (auto [import_vtable_entry_inst_id, local_vtable_entry_inst_id] :
       llvm::zip_equal(virtual_functions, lazy_virtual_functions)) {
    // Use LoadedImportRef for imported symbolic constant vtable entries so they
    // can carry attached constants necessary for applying specifics to these
    // constants when they are used.
    auto local_attached_constant_id =
        resolver.local_constant_values().GetAttached(
            local_vtable_entry_inst_id);
    if (local_attached_constant_id.is_symbolic()) {
      local_vtable_entry_inst_id = AddLoadedImportRef(
          resolver,
          GetSingletonType(resolver.local_context(),
                           SemIR::SpecificFunctionType::TypeInstId),
          import_vtable_entry_inst_id, local_attached_constant_id);
    }
  }

  auto class_id = SemIR::ClassId::None;
  if (class_const_inst.Is<SemIR::ClassType>()) {
    class_id = class_const_inst.As<SemIR::ClassType>().class_id;
  } else {
    auto generic_class_type =
        resolver.local_types().GetAs<SemIR::GenericClassType>(
            class_const_inst.type_id());
    class_id = generic_class_type.class_id;
  }

  auto new_vtable_id = resolver.local_vtables().Add(
      {{.class_id = class_id,
        .virtual_functions_id =
            resolver.local_inst_blocks().Add(lazy_virtual_functions)}});

  return ResolveResult::Deduplicated<SemIR::VtableDecl>(
      resolver, {.type_id = GetPointerType(resolver.local_context(),
                                           SemIR::VtableType::TypeInstId),
                 .vtable_id = new_vtable_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::VtablePtr inst) -> ResolveResult {
  auto specific_data = GetLocalSpecificData(resolver, inst.specific_id);

  auto vtable_const_id = GetLocalConstantId(
      resolver, resolver.import_classes()
                    .Get(resolver.import_vtables().Get(inst.vtable_id).class_id)
                    .vtable_decl_id);
  auto vtable_const_inst = resolver.local_insts().Get(
      resolver.local_constant_values().GetInstId(vtable_const_id));
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::VtablePtr>(
      resolver,
      {.type_id = GetPointerType(resolver.local_context(),
                                 SemIR::VtableType::TypeInstId),
       .vtable_id = vtable_const_inst.As<SemIR::VtableDecl>().vtable_id,
       .specific_id =
           GetOrAddLocalSpecific(resolver, inst.specific_id, specific_data)});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::FunctionType inst) -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto fn_val_id = GetLocalConstantInstId(
      resolver,
      resolver.import_functions().Get(inst.function_id).first_decl_id());

  auto specific_data = GetLocalSpecificData(resolver, inst.specific_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }
  auto fn_type_id = resolver.local_insts().Get(fn_val_id).type_id();
  return ResolveResult::Deduplicated<SemIR::FunctionType>(
      resolver, {.type_id = SemIR::TypeType::TypeId,
                 .function_id = resolver.local_types()
                                    .GetAs<SemIR::FunctionType>(fn_type_id)
                                    .function_id,
                 .specific_id = GetOrAddLocalSpecific(
                     resolver, inst.specific_id, specific_data)});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::FunctionTypeWithSelfType inst)
    -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto interface_function_type_id =
      GetLocalTypeInstId(resolver, inst.interface_function_type_id);
  auto self_id = GetLocalConstantInstId(resolver, inst.self_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::FunctionTypeWithSelfType>(
      resolver, {.type_id = SemIR::TypeType::TypeId,
                 .interface_function_type_id = interface_function_type_id,
                 .self_id = self_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::GenericClassType inst) -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto class_val_id = GetLocalConstantInstId(
      resolver,
      resolver.import_classes().Get(inst.class_id).first_owning_decl_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }
  auto class_val = resolver.local_insts().Get(class_val_id);
  CARBON_CHECK(
      resolver.local_types().Is<SemIR::GenericClassType>(class_val.type_id()));
  return ResolveResult::Done(
      resolver.local_types().GetConstantId(class_val.type_id()));
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::GenericInterfaceType inst)
    -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto interface_val_id = GetLocalConstantInstId(
      resolver,
      resolver.import_interfaces().Get(inst.interface_id).first_owning_decl_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }
  auto interface_val = resolver.local_insts().Get(interface_val_id);
  CARBON_CHECK(resolver.local_types().Is<SemIR::GenericInterfaceType>(
      interface_val.type_id()));
  return ResolveResult::Done(
      resolver.local_types().GetConstantId(interface_val.type_id()));
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::GenericNamedConstraintType inst)
    -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto constraint_val_id =
      GetLocalConstantInstId(resolver, resolver.import_named_constraints()
                                           .Get(inst.named_constraint_id)
                                           .first_owning_decl_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }
  auto constraint_val = resolver.local_insts().Get(constraint_val_id);
  CARBON_CHECK(resolver.local_types().Is<SemIR::GenericNamedConstraintType>(
      constraint_val.type_id()));
  return ResolveResult::Done(
      resolver.local_types().GetConstantId(constraint_val.type_id()));
}

// Make a declaration of an impl. This is done as a separate step from
// importing the impl definition in order to resolve cycles.
static auto MakeImplDeclaration(ImportContext& context,
                                const SemIR::Impl& import_impl,
                                SemIR::InstId witness_id)
    -> std::pair<SemIR::ImplId, SemIR::ConstantId> {
  SemIR::ImplDecl impl_decl = {.impl_id = SemIR::ImplId::None,
                               .decl_block_id = SemIR::InstBlockId::Empty};
  auto impl_decl_id = AddPlaceholderImportedInst(
      context, import_impl.latest_decl_id(), impl_decl);
  impl_decl.impl_id = context.local_impls().Add(
      {GetIncompleteLocalEntityBase(context, impl_decl_id, import_impl),
       {.self_id = SemIR::TypeInstId::None,
        .constraint_id = SemIR::TypeInstId::None,
        .interface = SemIR::SpecificInterface::None,
        .witness_id = witness_id,
        .scope_id = import_impl.is_complete() ? AddPlaceholderNameScope(context)
                                              : SemIR::NameScopeId::None,
        .is_final = import_impl.is_final}});

  // Write the impl ID into the ImplDecl.
  auto impl_const_id =
      ReplacePlaceholderImportedInst(context, impl_decl_id, impl_decl);
  return {impl_decl.impl_id, impl_const_id};
}

// Imports the definition of an impl.
static auto AddImplDefinition(ImportContext& context,
                              const SemIR::Impl& import_impl,
                              SemIR::Impl& new_impl) -> void {
  new_impl.definition_id = new_impl.first_owning_decl_id;
  new_impl.defined = true;

  if (import_impl.scope_id.has_value()) {
    auto& new_scope = context.local_name_scopes().Get(new_impl.scope_id);
    new_scope.Set(new_impl.first_owning_decl_id, SemIR::NameId::None,
                  new_impl.parent_scope_id);
    // Import the contents of the definition scope, if we might need it. Name
    // lookup is never performed into this scope by a user of the impl, so
    // this is only necessary in the same library that defined the impl, in
    // order to support defining members of the impl out of line in the impl
    // file when the impl is defined in the API file.
    // TODO: Check to see if this impl is owned by the API file, rather than
    // merely being imported into it.
    if (context.import_ir_id() == SemIR::ImportIRId::ApiForImpl) {
      const auto& import_scope =
          context.import_name_scopes().Get(import_impl.scope_id);

      // Push a block so that we can add scoped instructions to it.
      context.local_context().inst_block_stack().Push();
      AddNameScopeImportRefs(context, import_scope, new_scope);
      new_impl.body_block_id = context.local_context().inst_block_stack().Pop();
    }
  }
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::ImplDecl inst,
                                SemIR::ConstantId impl_const_id)
    -> ResolveResult {
  // TODO: This duplicates a lot of the handling of interfaces, classes, and
  // functions. Factor out the commonality.
  const auto& import_impl = resolver.import_impls().Get(inst.impl_id);
  auto specific_interface_data =
      GetLocalSpecificInterfaceData(resolver, import_impl.interface);
  SemIR::ImplId impl_id = SemIR::ImplId::None;
  if (!impl_const_id.has_value()) {
    if (resolver.HasNewWork()) {
      // This is the end of the first phase. Don't make a new impl yet if we
      // already have new work.
      return ResolveResult::Retry();
    }

    // On the second phase, create a forward declaration of the impl for any
    // recursive references.
    auto witness_id = AddImportRef(resolver, import_impl.witness_id);
    std::tie(impl_id, impl_const_id) =
        MakeImplDeclaration(resolver, import_impl, witness_id);
  } else {
    // On the third phase, compute the impl ID from the "constant value" of
    // the declaration, which is a reference to the created ImplDecl.
    auto impl_const_inst = resolver.local_insts().GetAs<SemIR::ImplDecl>(
        resolver.local_constant_values().GetInstId(impl_const_id));
    impl_id = impl_const_inst.impl_id;
  }

  // Load constants for the definition.
  auto parent_scope_id =
      GetLocalNameScopeId(resolver, import_impl.parent_scope_id);
  auto implicit_param_patterns = GetLocalInstBlockContents(
      resolver, import_impl.implicit_param_patterns_id);
  auto generic_data = GetLocalGenericData(resolver, import_impl.generic_id);
  auto self_const_id = GetLocalConstantId(resolver, import_impl.self_id);
  auto constraint_const_id =
      GetLocalConstantId(resolver, import_impl.constraint_id);
  auto& new_impl = resolver.local_impls().Get(impl_id);

  if (resolver.HasNewWork()) {
    return ResolveResult::Retry(impl_const_id, new_impl.first_decl_id());
  }

  new_impl.parent_scope_id = parent_scope_id;
  new_impl.implicit_param_patterns_id = GetLocalCanonicalInstBlockId(
      resolver, import_impl.implicit_param_patterns_id,
      implicit_param_patterns);
  SetGenericData(resolver, import_impl.generic_id, new_impl.generic_id,
                 generic_data);

  // Create instructions for self and constraint to hold the symbolic constant
  // value for a generic impl.
  new_impl.self_id =
      AddLoadedImportRefForType(resolver, import_impl.self_id, self_const_id);
  new_impl.constraint_id = AddLoadedImportRefForType(
      resolver, import_impl.constraint_id, constraint_const_id);
  new_impl.interface = GetLocalSpecificInterface(
      resolver, import_impl.interface, specific_interface_data);
  if (import_impl.is_complete()) {
    AddImplDefinition(resolver, import_impl, new_impl);
  }

  // If the `impl` is declared in the API file corresponding to the current
  // file, add this to impl lookup so that it can be found by redeclarations
  // in the current file.
  if (resolver.import_ir_id() == SemIR::ImportIRId::ApiForImpl) {
    resolver.local_impls().GetOrAddLookupBucket(new_impl).push_back(impl_id);
  }

  return ResolveResult::Done(impl_const_id, new_impl.first_decl_id());
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::RequireImplsDecl inst) -> ResolveResult {
  const auto& import_require =
      resolver.import_require_impls().Get(inst.require_impls_id);

  // Load dependent constants.
  auto parent_scope_id =
      GetLocalNameScopeId(resolver, import_require.parent_scope_id);
  auto generic_data = GetLocalGenericData(resolver, import_require.generic_id);
  auto self_const_id = GetLocalConstantId(resolver, import_require.self_id);
  auto facet_type_const_id =
      GetLocalConstantId(resolver, import_require.facet_type_inst_id);

  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  // Make the decl and structure with placeholder values to be filled in.
  SemIR::RequireImplsDecl require_decl = {
      .require_impls_id = SemIR::RequireImplsId::None,
      .decl_block_id = SemIR::InstBlockId::Empty};
  auto require_decl_id = AddPlaceholderImportedInst(
      resolver, import_require.decl_id, require_decl);
  auto require_impls_id = resolver.local_require_impls().Add(
      {.self_id = SemIR::TypeInstId::None,
       .facet_type_inst_id = SemIR::TypeInstId::None,
       .facet_type_id = SemIR::FacetTypeId::None,
       .decl_id = require_decl_id,
       .parent_scope_id = SemIR::NameScopeId::None,
       .generic_id = MakeIncompleteGeneric(resolver, require_decl_id,
                                           import_require.generic_id)});

  // Write the RequireImplsId into the RequireImplsDecl.
  require_decl.require_impls_id = require_impls_id;
  auto require_decl_const_id =
      ReplacePlaceholderImportedInst(resolver, require_decl_id, require_decl);

  // Fill in the RequireImpls structure.
  auto& new_require = resolver.local_require_impls().Get(require_impls_id);
  new_require.self_id = AddLoadedImportRefForType(
      resolver, import_require.self_id, self_const_id);
  new_require.facet_type_inst_id = AddLoadedImportRefForType(
      resolver, import_require.facet_type_inst_id, facet_type_const_id);
  auto new_canonical_facet_type_inst_id =
      resolver.local_constant_values().GetConstantInstId(
          new_require.facet_type_inst_id);
  auto new_canonical_facet_type =
      resolver.local_insts().GetAs<SemIR::FacetType>(
          new_canonical_facet_type_inst_id);
  new_require.facet_type_id = new_canonical_facet_type.facet_type_id;
  new_require.extend_self = import_require.extend_self;
  new_require.parent_scope_id = parent_scope_id;

  SetGenericData(resolver, import_require.generic_id, new_require.generic_id,
                 generic_data);

  return ResolveResult::Done(require_decl_const_id, require_decl_id);
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::ImportRefLoaded /*inst*/,
                                SemIR::InstId inst_id) -> ResolveResult {
  // Return the constant for the instruction of the imported constant.
  auto constant_id = resolver.import_constant_values().Get(inst_id);
  CARBON_CHECK(constant_id.has_value(),
               "Loaded import ref has no constant value");
  if (!constant_id.is_constant()) {
    resolver.local_context().TODO(
        inst_id, "Non-constant ImportRefLoaded (comes up with var)");
    return ResolveResult::Done(constant_id);
  }

  auto new_constant_id = GetLocalConstantId(resolver, constant_id);
  return ResolveResult::RetryOrDone(resolver, new_constant_id);
}

// Make a declaration of an interface. This is done as a separate step from
// importing the interface definition in order to resolve cycles.
static auto MakeInterfaceDecl(ImportContext& context,
                              const SemIR::Interface& import_interface,
                              SemIR::SpecificId enclosing_specific_id)
    -> std::pair<SemIR::InterfaceId, SemIR::ConstantId> {
  SemIR::InterfaceDecl interface_decl = {
      .type_id = SemIR::TypeType::TypeId,
      .interface_id = SemIR::InterfaceId::None,
      .decl_block_id = SemIR::InstBlockId::Empty};
  auto interface_decl_id = AddPlaceholderImportedInst(
      context, import_interface.first_owning_decl_id, interface_decl);

  // Start with an incomplete interface.
  interface_decl.interface_id = context.local_interfaces().Add(
      {GetIncompleteLocalEntityBase(context, interface_decl_id,
                                    import_interface),
       {.scope_id = import_interface.is_complete()
                        ? AddPlaceholderNameScope(context)
                        : SemIR::NameScopeId::None}});

  if (import_interface.has_parameters()) {
    interface_decl.type_id = GetGenericInterfaceType(
        context.local_context(), interface_decl.interface_id,
        enclosing_specific_id);
  }

  // Write the interface ID into the InterfaceDecl.
  auto interface_const_id = ReplacePlaceholderImportedInst(
      context, interface_decl_id, interface_decl);
  return {interface_decl.interface_id, interface_const_id};
}

// Imports the definition for an interface that has been imported as a forward
// declaration.
static auto AddInterfaceDefinition(ImportContext& context,
                                   const SemIR::Interface& import_interface,
                                   SemIR::Interface& new_interface,
                                   SemIR::InstId self_param_id) -> void {
  auto& new_scope = context.local_name_scopes().Get(new_interface.scope_id);
  const auto& import_scope =
      context.import_name_scopes().Get(import_interface.scope_id);

  // Push a block so that we can add scoped instructions to it.
  context.local_context().inst_block_stack().Push();
  InitializeNameScopeAndImportRefs(
      context, import_scope, new_scope, new_interface.first_owning_decl_id,
      SemIR::NameId::None, new_interface.parent_scope_id);
  new_scope.set_is_interface_definition();
  new_interface.associated_entities_id = AddAssociatedEntities(
      context, new_interface.scope_id, import_interface.associated_entities_id);
  new_interface.body_block_id =
      context.local_context().inst_block_stack().Pop();
  new_interface.self_param_id = self_param_id;

  CARBON_CHECK(import_scope.extended_scopes().empty(),
               "Interfaces don't currently have extended scopes to support.");
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::InterfaceDecl inst,
                                SemIR::ConstantId interface_const_id)
    -> ResolveResult {
  const auto& import_interface =
      resolver.import_interfaces().Get(inst.interface_id);

  auto interface_id = SemIR::InterfaceId::None;
  if (!interface_const_id.has_value()) {
    auto import_specific_id = SemIR::SpecificId::None;
    if (auto import_generic_interface_type =
            resolver.import_types().TryGetAs<SemIR::GenericInterfaceType>(
                inst.type_id)) {
      import_specific_id = import_generic_interface_type->enclosing_specific_id;
    }
    auto specific_data = GetLocalSpecificData(resolver, import_specific_id);
    if (resolver.HasNewWork()) {
      // This is the end of the first phase. Don't make a new interface yet if
      // we already have new work.
      return ResolveResult::Retry();
    }

    // On the second phase, create a forward declaration of the interface.
    auto enclosing_specific_id =
        GetOrAddLocalSpecific(resolver, import_specific_id, specific_data);
    std::tie(interface_id, interface_const_id) =
        MakeInterfaceDecl(resolver, import_interface, enclosing_specific_id);
  } else {
    // On the third phase, compute the interface ID from the constant value of
    // the declaration.
    auto interface_const_inst = resolver.local_insts().Get(
        resolver.local_constant_values().GetInstId(interface_const_id));
    if (auto facet_type = interface_const_inst.TryAs<SemIR::FacetType>()) {
      const SemIR::FacetTypeInfo& facet_type_info =
          resolver.local_facet_types().Get(facet_type->facet_type_id);
      auto single = facet_type_info.TryAsSingleExtend();
      CARBON_CHECK(single);
      interface_id = std::get<SemIR::SpecificInterface>(*single).interface_id;
    } else {
      auto generic_interface_type =
          resolver.local_types().GetAs<SemIR::GenericInterfaceType>(
              interface_const_inst.type_id());
      interface_id = generic_interface_type.interface_id;
    }
  }

  auto parent_scope_id =
      GetLocalNameScopeId(resolver, import_interface.parent_scope_id);
  auto implicit_param_patterns = GetLocalInstBlockContents(
      resolver, import_interface.implicit_param_patterns_id);
  auto param_patterns =
      GetLocalInstBlockContents(resolver, import_interface.param_patterns_id);
  auto generic_data =
      GetLocalGenericData(resolver, import_interface.generic_id);
  auto require_impls = GetLocalRequireImplsBlockContents(
      resolver, import_interface.require_impls_block_id);

  std::optional<SemIR::InstId> self_param_id;
  if (import_interface.is_complete()) {
    self_param_id =
        GetLocalConstantInstId(resolver, import_interface.self_param_id);
  }
  auto& new_interface = resolver.local_interfaces().Get(interface_id);

  if (resolver.HasNewWork()) {
    return ResolveResult::Retry(interface_const_id,
                                new_interface.first_decl_id());
  }

  new_interface.parent_scope_id = parent_scope_id;
  new_interface.implicit_param_patterns_id = GetLocalCanonicalInstBlockId(
      resolver, import_interface.implicit_param_patterns_id,
      implicit_param_patterns);
  new_interface.param_patterns_id = GetLocalCanonicalInstBlockId(
      resolver, import_interface.param_patterns_id, param_patterns);
  new_interface.require_impls_block_id = GetLocalCanonicalRequireImplsBlockId(
      resolver, import_interface.require_impls_block_id, require_impls);
  SetGenericData(resolver, import_interface.generic_id,
                 new_interface.generic_id, generic_data);

  if (import_interface.is_complete()) {
    CARBON_CHECK(self_param_id);
    AddInterfaceDefinition(resolver, import_interface, new_interface,
                           *self_param_id);
  }
  return ResolveResult::Done(interface_const_id, new_interface.first_decl_id());
}

// Make a declaration of a named constraint. This is done as a separate step
// from importing the constraint definition in order to resolve cycles.
static auto MakeNamedConstraintDecl(
    ImportContext& context,
    const SemIR::NamedConstraint& import_named_constraint,
    SemIR::SpecificId enclosing_specific_id)
    -> std::pair<SemIR::NamedConstraintId, SemIR::ConstantId> {
  SemIR::NamedConstraintDecl named_constraint_decl = {
      .type_id = SemIR::TypeType::TypeId,
      .named_constraint_id = SemIR::NamedConstraintId::None,
      .decl_block_id = SemIR::InstBlockId::Empty};
  auto named_constraint_decl_id = AddPlaceholderImportedInst(
      context, import_named_constraint.first_owning_decl_id,
      named_constraint_decl);

  // Start with an incomplete interface.
  named_constraint_decl.named_constraint_id =
      context.local_named_constraints().Add(
          {GetIncompleteLocalEntityBase(context, named_constraint_decl_id,
                                        import_named_constraint),
           {.scope_id = import_named_constraint.is_complete()
                            ? AddPlaceholderNameScope(context)
                            : SemIR::NameScopeId::None}});

  if (import_named_constraint.has_parameters()) {
    named_constraint_decl.type_id = GetGenericNamedConstraintType(
        context.local_context(), named_constraint_decl.named_constraint_id,
        enclosing_specific_id);
  }

  // Write the interface ID into the InterfaceDecl.
  auto interface_const_id = ReplacePlaceholderImportedInst(
      context, named_constraint_decl_id, named_constraint_decl);
  return {named_constraint_decl.named_constraint_id, interface_const_id};
}

// Imports the definition for a named constraint that has been imported as a
// forward declaration.
static auto AddNamedConstraintDefinition(
    ImportContext& context,
    const SemIR::NamedConstraint& import_named_constraint,
    SemIR::NamedConstraint& new_named_constraint, SemIR::InstId self_param_id)
    -> void {
  auto& new_scope =
      context.local_name_scopes().Get(new_named_constraint.scope_id);
  const auto& import_scope =
      context.import_name_scopes().Get(import_named_constraint.scope_id);

  // Push a block so that we can add scoped instructions to it.
  context.local_context().inst_block_stack().Push();
  InitializeNameScopeAndImportRefs(context, import_scope, new_scope,
                                   new_named_constraint.first_owning_decl_id,
                                   SemIR::NameId::None,
                                   new_named_constraint.parent_scope_id);
  new_named_constraint.body_block_id =
      context.local_context().inst_block_stack().Pop();
  new_named_constraint.self_param_id = self_param_id;
  new_named_constraint.complete = import_named_constraint.complete;

  CARBON_CHECK(
      import_scope.extended_scopes().empty(),
      "Named constraints don't currently have extended scopes to support.");
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::NamedConstraintDecl inst,
                                SemIR::ConstantId named_constraint_const_id)
    -> ResolveResult {
  const auto& import_named_constraint =
      resolver.import_named_constraints().Get(inst.named_constraint_id);

  auto named_constraint_id = SemIR::NamedConstraintId::None;
  if (!named_constraint_const_id.has_value()) {
    auto import_specific_id = SemIR::SpecificId::None;
    if (auto import_generic_named_constraint_type =
            resolver.import_types().TryGetAs<SemIR::GenericNamedConstraintType>(
                inst.type_id)) {
      import_specific_id =
          import_generic_named_constraint_type->enclosing_specific_id;
    }
    auto specific_data = GetLocalSpecificData(resolver, import_specific_id);
    if (resolver.HasNewWork()) {
      // This is the end of the first phase. Don't make a new interface yet if
      // we already have new work.
      return ResolveResult::Retry();
    }

    // On the second phase, create a forward declaration of the interface.
    auto enclosing_specific_id =
        GetOrAddLocalSpecific(resolver, import_specific_id, specific_data);
    std::tie(named_constraint_id, named_constraint_const_id) =
        MakeNamedConstraintDecl(resolver, import_named_constraint,
                                enclosing_specific_id);
  } else {
    // On the third phase, compute the interface ID from the constant value of
    // the declaration.
    auto named_constraint_const_inst = resolver.local_insts().Get(
        resolver.local_constant_values().GetInstId(named_constraint_const_id));
    if (auto facet_type =
            named_constraint_const_inst.TryAs<SemIR::FacetType>()) {
      const SemIR::FacetTypeInfo& facet_type_info =
          resolver.local_facet_types().Get(facet_type->facet_type_id);
      auto single = facet_type_info.TryAsSingleExtend();
      CARBON_CHECK(single);
      named_constraint_id =
          std::get<SemIR::SpecificNamedConstraint>(*single).named_constraint_id;
    } else {
      auto generic_named_constraint_type =
          resolver.local_types().GetAs<SemIR::GenericNamedConstraintType>(
              named_constraint_const_inst.type_id());
      named_constraint_id = generic_named_constraint_type.named_constraint_id;
    }
  }

  auto parent_scope_id =
      GetLocalNameScopeId(resolver, import_named_constraint.parent_scope_id);
  auto implicit_param_patterns = GetLocalInstBlockContents(
      resolver, import_named_constraint.implicit_param_patterns_id);
  auto param_patterns = GetLocalInstBlockContents(
      resolver, import_named_constraint.param_patterns_id);
  auto generic_data =
      GetLocalGenericData(resolver, import_named_constraint.generic_id);
  auto require_impls = GetLocalRequireImplsBlockContents(
      resolver, import_named_constraint.require_impls_block_id);

  std::optional<SemIR::InstId> self_param_id;
  if (import_named_constraint.is_complete()) {
    self_param_id =
        GetLocalConstantInstId(resolver, import_named_constraint.self_param_id);
  }
  auto& new_named_constraint =
      resolver.local_named_constraints().Get(named_constraint_id);

  if (resolver.HasNewWork()) {
    return ResolveResult::Retry(named_constraint_const_id,
                                new_named_constraint.first_decl_id());
  }

  new_named_constraint.parent_scope_id = parent_scope_id;
  new_named_constraint.implicit_param_patterns_id =
      GetLocalCanonicalInstBlockId(
          resolver, import_named_constraint.implicit_param_patterns_id,
          implicit_param_patterns);
  new_named_constraint.param_patterns_id = GetLocalCanonicalInstBlockId(
      resolver, import_named_constraint.param_patterns_id, param_patterns);
  new_named_constraint.require_impls_block_id =
      GetLocalCanonicalRequireImplsBlockId(
          resolver, import_named_constraint.require_impls_block_id,
          require_impls);
  SetGenericData(resolver, import_named_constraint.generic_id,
                 new_named_constraint.generic_id, generic_data);

  if (import_named_constraint.is_complete()) {
    CARBON_CHECK(self_param_id);
    AddNamedConstraintDefinition(resolver, import_named_constraint,
                                 new_named_constraint, *self_param_id);
  }
  return ResolveResult::Done(named_constraint_const_id,
                             new_named_constraint.first_decl_id());
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::FacetAccessType inst) -> ResolveResult {
  auto facet_value_inst_id =
      GetLocalConstantInstId(resolver, inst.facet_value_inst_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::FacetAccessType>(
      resolver, {.type_id = SemIR::TypeType::TypeId,
                 .facet_value_inst_id = facet_value_inst_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::FacetType inst) -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);

  const SemIR::FacetTypeInfo& import_facet_type_info =
      resolver.import_facet_types().Get(inst.facet_type_id);
  for (auto interface : import_facet_type_info.extend_constraints) {
    // We discard this here and recompute it below instead of saving it to avoid
    // allocations.
    GetLocalSpecificInterfaceData(resolver, interface);
  }
  for (auto interface : import_facet_type_info.self_impls_constraints) {
    // We discard this here and recompute it below instead of saving it to avoid
    // allocations.
    GetLocalSpecificInterfaceData(resolver, interface);
  }
  for (auto constraint : import_facet_type_info.extend_named_constraints) {
    // We discard this here and recompute it below instead of saving it to avoid
    // allocations.
    GetLocalSpecificNamedConstraintData(resolver, constraint);
  }
  for (auto constraint : import_facet_type_info.self_impls_named_constraints) {
    // We discard this here and recompute it below instead of saving it to avoid
    // allocations.
    GetLocalSpecificNamedConstraintData(resolver, constraint);
  }
  for (auto rewrite : import_facet_type_info.rewrite_constraints) {
    GetLocalConstantInstId(resolver, rewrite.lhs_id);
    GetLocalConstantInstId(resolver, rewrite.rhs_id);
  }
  // TODO: Import named constraints in the facet type.
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  SemIR::FacetTypeInfo local_facet_type_info = {
      .builtin_constraint_mask = import_facet_type_info.builtin_constraint_mask,
      // TODO: Also process the other requirements.
      .other_requirements = import_facet_type_info.other_requirements};
  local_facet_type_info.extend_constraints.reserve(
      import_facet_type_info.extend_constraints.size());
  for (auto interface : import_facet_type_info.extend_constraints) {
    auto data = GetLocalSpecificInterfaceData(resolver, interface);
    local_facet_type_info.extend_constraints.push_back(
        GetLocalSpecificInterface(resolver, interface, data));
  }
  local_facet_type_info.self_impls_constraints.reserve(
      import_facet_type_info.self_impls_constraints.size());
  for (auto interface : import_facet_type_info.self_impls_constraints) {
    auto data = GetLocalSpecificInterfaceData(resolver, interface);
    local_facet_type_info.self_impls_constraints.push_back(
        GetLocalSpecificInterface(resolver, interface, data));
  }
  local_facet_type_info.extend_named_constraints.reserve(
      import_facet_type_info.extend_named_constraints.size());
  for (auto constraint : import_facet_type_info.extend_named_constraints) {
    auto data = GetLocalSpecificNamedConstraintData(resolver, constraint);
    local_facet_type_info.extend_named_constraints.push_back(
        GetLocalSpecificNamedConstraint(resolver, constraint, data));
  }
  local_facet_type_info.self_impls_named_constraints.reserve(
      import_facet_type_info.self_impls_named_constraints.size());
  for (auto constraint : import_facet_type_info.self_impls_named_constraints) {
    auto data = GetLocalSpecificNamedConstraintData(resolver, constraint);
    local_facet_type_info.self_impls_named_constraints.push_back(
        GetLocalSpecificNamedConstraint(resolver, constraint, data));
  }
  local_facet_type_info.rewrite_constraints.reserve(
      import_facet_type_info.rewrite_constraints.size());
  for (auto rewrite : import_facet_type_info.rewrite_constraints) {
    local_facet_type_info.rewrite_constraints.push_back(
        {.lhs_id = GetLocalConstantInstId(resolver, rewrite.lhs_id),
         .rhs_id = GetLocalConstantInstId(resolver, rewrite.rhs_id)});
  }
  SemIR::FacetTypeId facet_type_id =
      resolver.local_facet_types().Add(std::move(local_facet_type_info));
  return ResolveResult::Deduplicated<SemIR::FacetType>(
      resolver,
      {.type_id = SemIR::TypeType::TypeId, .facet_type_id = facet_type_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::FacetValue inst) -> ResolveResult {
  auto type_id = GetLocalConstantId(resolver, inst.type_id);
  auto type_inst_id = GetLocalTypeInstId(resolver, inst.type_inst_id);
  auto witnesses = GetLocalInstBlockContents(resolver, inst.witnesses_block_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::FacetValue>(
      resolver,
      {.type_id =
           resolver.local_context().types().GetTypeIdForTypeConstantId(type_id),
       .type_inst_id = type_inst_id,
       .witnesses_block_id = GetLocalCanonicalInstBlockId(
           resolver, inst.witnesses_block_id, witnesses)});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::LookupImplWitness inst)
    -> ResolveResult {
  CARBON_CHECK(resolver.import_types().GetInstId(inst.type_id) ==
               SemIR::WitnessType::TypeInstId);

  auto query_self_inst_id =
      GetLocalConstantInstId(resolver, inst.query_self_inst_id);

  const auto& import_specific_interface =
      resolver.import_specific_interfaces().Get(
          inst.query_specific_interface_id);
  auto data =
      GetLocalSpecificInterfaceData(resolver, import_specific_interface);

  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  auto specific_interface =
      GetLocalSpecificInterface(resolver, import_specific_interface, data);
  auto query_specific_interface_id =
      resolver.local_specific_interfaces().Add(specific_interface);
  return ResolveResult::Deduplicated<SemIR::LookupImplWitness>(
      resolver, {.type_id = GetSingletonType(resolver.local_context(),
                                             SemIR::WitnessType::TypeInstId),
                 .query_self_inst_id = query_self_inst_id,
                 .query_specific_interface_id = query_specific_interface_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::ImplWitness inst) -> ResolveResult {
  CARBON_CHECK(resolver.import_types().GetInstId(inst.type_id) ==
               SemIR::WitnessType::TypeInstId);

  auto specific_data = GetLocalSpecificData(resolver, inst.specific_id);
  auto witness_table_id =
      GetLocalConstantInstId(resolver, inst.witness_table_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  auto specific_id =
      GetOrAddLocalSpecific(resolver, inst.specific_id, specific_data);
  return ResolveResult::Deduplicated<SemIR::ImplWitness>(
      resolver, {.type_id = GetSingletonType(resolver.local_context(),
                                             SemIR::WitnessType::TypeInstId),
                 .witness_table_id = witness_table_id,
                 .specific_id = specific_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::ImplWitnessAccess inst)
    -> ResolveResult {
  auto type_id = GetLocalConstantId(resolver, inst.type_id);
  auto witness_id = GetLocalConstantInstId(resolver, inst.witness_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::ImplWitnessAccess>(
      resolver,
      {.type_id =
           resolver.local_context().types().GetTypeIdForTypeConstantId(type_id),
       .witness_id = witness_id,
       .index = inst.index});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::ImplWitnessTable inst,
                                SemIR::InstId import_inst_id) -> ResolveResult {
  const auto& import_impl = resolver.import_impls().Get(inst.impl_id);
  auto import_decl_inst_id = import_impl.first_decl_id();
  auto local_decl_inst_id =
      GetLocalConstantInstId(resolver, import_decl_inst_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  auto impl_decl =
      resolver.local_insts().GetAs<SemIR::ImplDecl>(local_decl_inst_id);
  auto impl_id = impl_decl.impl_id;
  auto elements_id = GetLocalImportRefInstBlock(resolver, inst.elements_id);

  // Create a corresponding instruction to represent the table.
  return ResolveResult::Unique<SemIR::ImplWitnessTable>(
      resolver, import_inst_id,
      {.elements_id = elements_id, .impl_id = impl_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::IntValue inst) -> ResolveResult {
  auto type_id = GetLocalConstantId(resolver, inst.type_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  // We can directly reuse the value IDs across file IRs. Otherwise, we need
  // to add a new canonical int in this IR.
  auto int_id = inst.int_id.is_embedded_value()
                    ? inst.int_id
                    : resolver.local_ints().AddSigned(
                          resolver.import_ints().Get(inst.int_id));

  return ResolveResult::Deduplicated<SemIR::IntValue>(
      resolver,
      {.type_id =
           resolver.local_context().types().GetTypeIdForTypeConstantId(type_id),
       .int_id = int_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::IntType inst) -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto bit_width_id = GetLocalConstantInstId(resolver, inst.bit_width_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::IntType>(
      resolver, {.type_id = SemIR::TypeType::TypeId,
                 .int_kind = inst.int_kind,
                 .bit_width_id = bit_width_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::Namespace inst,
                                SemIR::InstId import_inst_id) -> ResolveResult {
  const auto& name_scope =
      resolver.import_name_scopes().Get(inst.name_scope_id);
  // A package from a different file becomes a child of the package here, as it
  // would be if it were imported.
  auto parent_scope_id =
      inst.name_scope_id == SemIR::NameScopeId::Package
          ? SemIR::NameScopeId::Package
          : GetLocalNameScopeId(resolver, name_scope.parent_scope_id());

  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  auto namespace_type_id = GetSingletonType(resolver.local_context(),
                                            SemIR::NamespaceType::TypeInstId);
  auto namespace_decl =
      SemIR::Namespace{.type_id = namespace_type_id,
                       .name_scope_id = SemIR::NameScopeId::None,
                       .import_id = SemIR::AbsoluteInstId::None};
  auto inst_id =
      AddPlaceholderImportedInst(resolver, import_inst_id, namespace_decl);

  auto name_id = GetLocalNameId(resolver, name_scope.name_id());
  namespace_decl.name_scope_id =
      resolver.local_name_scopes().Add(inst_id, name_id, parent_scope_id);
  // Namespaces from this package are eagerly imported, so anything we load here
  // must be a closed import.
  resolver.local_name_scopes()
      .Get(namespace_decl.name_scope_id)
      .set_is_closed_import(true);
  auto namespace_const_id =
      ReplacePlaceholderImportedInst(resolver, inst_id, namespace_decl);
  return {.const_id = namespace_const_id};
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::PatternType inst) -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto scrutinee_type_inst_id =
      GetLocalTypeInstId(resolver, inst.scrutinee_type_inst_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::PatternType>(
      resolver, {.type_id = SemIR::TypeType::TypeId,
                 .scrutinee_type_inst_id = scrutinee_type_inst_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::PointerType inst) -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto pointee_id = GetLocalTypeInstId(resolver, inst.pointee_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::PointerType>(
      resolver, {.type_id = SemIR::TypeType::TypeId, .pointee_id = pointee_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::RequireCompleteType inst)
    -> ResolveResult {
  CARBON_CHECK(resolver.import_types().GetInstId(inst.type_id) ==
               SemIR::WitnessType::TypeInstId);

  auto complete_type_inst_id =
      GetLocalTypeInstId(resolver, inst.complete_type_inst_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::RequireCompleteType>(
      resolver, {.type_id = GetSingletonType(resolver.local_context(),
                                             SemIR::WitnessType::TypeInstId),
                 .complete_type_inst_id = complete_type_inst_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::ReturnSlotPattern inst,
                                SemIR::InstId import_inst_id) -> ResolveResult {
  auto type_const_id = GetLocalConstantId(resolver, inst.type_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Unique<SemIR::ReturnSlotPattern>(
      resolver, import_inst_id,
      {.type_id = resolver.local_context().types().GetTypeIdForTypeConstantId(
           type_const_id),
       .type_inst_id = SemIR::TypeInstId::None});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::SpecificFunction inst) -> ResolveResult {
  auto type_const_id = GetLocalConstantId(resolver, inst.type_id);
  auto callee_id = GetLocalConstantInstId(resolver, inst.callee_id);
  auto specific_data = GetLocalSpecificData(resolver, inst.specific_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  auto type_id = resolver.local_context().types().GetTypeIdForTypeConstantId(
      type_const_id);
  auto specific_id =
      GetOrAddLocalSpecific(resolver, inst.specific_id, specific_data);
  return ResolveResult::Deduplicated<SemIR::SpecificFunction>(
      resolver,
      {.type_id = type_id, .callee_id = callee_id, .specific_id = specific_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::SpecificImplFunction inst)
    -> ResolveResult {
  auto callee_id = GetLocalConstantInstId(resolver, inst.callee_id);
  auto specific_data = GetLocalSpecificData(resolver, inst.specific_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  auto type_id = GetSingletonType(resolver.local_context(),
                                  SemIR::SpecificFunctionType::TypeInstId);
  auto specific_id =
      GetOrAddLocalSpecific(resolver, inst.specific_id, specific_data);
  return ResolveResult::Deduplicated<SemIR::SpecificImplFunction>(
      resolver,
      {.type_id = type_id, .callee_id = callee_id, .specific_id = specific_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::StructAccess inst) -> ResolveResult {
  auto type_id = GetLocalConstantId(resolver, inst.type_id);
  auto struct_id = GetLocalConstantInstId(resolver, inst.struct_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  // A `struct_access` constant requires its struct operand to have a complete
  // type.
  CompleteTypeOrCheckFail(resolver.local_context(),
                          resolver.local_insts().Get(struct_id).type_id());

  return ResolveResult::Deduplicated<SemIR::StructAccess>(
      resolver,
      {.type_id =
           resolver.local_context().types().GetTypeIdForTypeConstantId(type_id),
       .struct_id = struct_id,
       .index = inst.index});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::StructType inst) -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto orig_fields = resolver.import_struct_type_fields().Get(inst.fields_id);
  llvm::SmallVector<SemIR::TypeInstId> field_type_inst_ids;
  field_type_inst_ids.reserve(orig_fields.size());
  for (auto field : orig_fields) {
    field_type_inst_ids.push_back(
        GetLocalTypeInstId(resolver, field.type_inst_id));
  }
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  // Prepare a vector of fields for GetStructType.
  llvm::SmallVector<SemIR::StructTypeField> new_fields;
  new_fields.reserve(orig_fields.size());
  for (auto [orig_field, field_type_inst_id] :
       llvm::zip_equal(orig_fields, field_type_inst_ids)) {
    auto name_id = GetLocalNameId(resolver, orig_field.name_id);
    new_fields.push_back(
        {.name_id = name_id, .type_inst_id = field_type_inst_id});
  }

  return ResolveResult::Deduplicated<SemIR::StructType>(
      resolver, {.type_id = SemIR::TypeType::TypeId,
                 .fields_id = resolver.local_struct_type_fields().AddCanonical(
                     new_fields)});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::StructValue inst) -> ResolveResult {
  auto type_id = GetLocalConstantId(resolver, inst.type_id);
  auto elems = GetLocalInstBlockContents(resolver, inst.elements_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::StructValue>(
      resolver,
      {.type_id =
           resolver.local_context().types().GetTypeIdForTypeConstantId(type_id),
       .elements_id =
           GetLocalCanonicalInstBlockId(resolver, inst.elements_id, elems)});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::SymbolicBindingType inst)
    -> ResolveResult {
  auto facet_value_inst_id =
      GetLocalConstantInstId(resolver, inst.facet_value_inst_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  auto entity_name_id =
      GetLocalSymbolicEntityNameId(resolver, inst.entity_name_id);
  return ResolveResult::Deduplicated<SemIR::SymbolicBindingType>(
      resolver, {.type_id = SemIR::TypeType::TypeId,
                 .entity_name_id = entity_name_id,
                 .facet_value_inst_id = facet_value_inst_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::TupleAccess inst) -> ResolveResult {
  auto type_id = GetLocalConstantId(resolver, inst.type_id);
  auto tuple_id = GetLocalConstantInstId(resolver, inst.tuple_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  // A `tuple_access` constant requires its struct operand to have a complete
  // type.
  CompleteTypeOrCheckFail(resolver.local_context(),
                          resolver.local_insts().Get(tuple_id).type_id());

  return ResolveResult::Deduplicated<SemIR::TupleAccess>(
      resolver,
      {.type_id =
           resolver.local_context().types().GetTypeIdForTypeConstantId(type_id),
       .tuple_id = tuple_id,
       .index = inst.index});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::TuplePattern inst,
                                SemIR::InstId import_inst_id) -> ResolveResult {
  auto type_const_id = GetLocalConstantId(resolver, inst.type_id);
  auto elements = GetLocalInstBlockContents(resolver, inst.elements_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Unique<SemIR::TuplePattern>(
      resolver, import_inst_id,
      {.type_id = resolver.local_context().types().GetTypeIdForTypeConstantId(
           type_const_id),
       .elements_id =
           GetLocalCanonicalInstBlockId(resolver, inst.elements_id, elements)});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::TupleType inst) -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);

  auto orig_type_inst_ids =
      resolver.import_inst_blocks().Get(inst.type_elements_id);
  // TODO: It might be nice to make the `InstBlock` in `TupleType` record in the
  // type system that its holding `TypeInstId` elements.
  llvm::SmallVector<SemIR::InstId> type_inst_ids;
  type_inst_ids.reserve(orig_type_inst_ids.size());
  for (auto elem_type_inst_id :
       resolver.import_ir().types().GetBlockAsTypeInstIds(orig_type_inst_ids)) {
    type_inst_ids.push_back(GetLocalTypeInstId(resolver, elem_type_inst_id));
  }
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::TupleType>(
      resolver, {.type_id = SemIR::TypeType::TypeId,
                 .type_elements_id = GetLocalCanonicalInstBlockId(
                     resolver, inst.type_elements_id, type_inst_ids)});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::TupleValue inst) -> ResolveResult {
  auto type_id = GetLocalConstantId(resolver, inst.type_id);
  auto elems = GetLocalInstBlockContents(resolver, inst.elements_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::TupleValue>(
      resolver,
      {.type_id =
           resolver.local_context().types().GetTypeIdForTypeConstantId(type_id),
       .elements_id =
           GetLocalCanonicalInstBlockId(resolver, inst.elements_id, elems)});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::UnboundElementType inst)
    -> ResolveResult {
  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
  auto class_const_inst_id =
      GetLocalTypeInstId(resolver, inst.class_type_inst_id);
  auto elem_const_inst_id =
      GetLocalTypeInstId(resolver, inst.element_type_inst_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Deduplicated<SemIR::UnboundElementType>(
      resolver, {.type_id = SemIR::TypeType::TypeId,
                 .class_type_inst_id = class_const_inst_id,
                 .element_type_inst_id = elem_const_inst_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::VarPattern inst,
                                SemIR::InstId import_inst_id) -> ResolveResult {
  auto type_const_id = GetLocalConstantId(resolver, inst.type_id);
  auto subpattern_id = GetLocalConstantInstId(resolver, inst.subpattern_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Unique<SemIR::VarPattern>(
      resolver, import_inst_id,
      {.type_id = resolver.local_context().types().GetTypeIdForTypeConstantId(
           type_const_id),
       .subpattern_id = subpattern_id});
}

static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                SemIR::VarStorage inst,
                                SemIR::InstId import_inst_id) -> ResolveResult {
  auto type_const_id = GetLocalConstantId(resolver, inst.type_id);
  auto pattern_id = GetLocalConstantInstId(resolver, inst.pattern_id);
  if (resolver.HasNewWork()) {
    return ResolveResult::Retry();
  }

  return ResolveResult::Unique<SemIR::VarStorage>(
      resolver, import_inst_id,
      {.type_id = resolver.local_context().types().GetTypeIdForTypeConstantId(
           type_const_id),
       .pattern_id = pattern_id});
}

// Tries to resolve the InstId, returning a canonical constant when ready, or
// `None` if more has been added to the stack. This is the same as
// TryResolveInst, except that it may resolve symbolic constants as canonical
// constants instead of as constants associated with a particular generic.
static auto TryResolveInstCanonical(ImportRefResolver& resolver,
                                    SemIR::InstId inst_id,
                                    SemIR::ConstantId const_id)
    -> ResolveResult {
  // These instruction types are imported across multiple phases to arrive at
  // their constant value. We can't just import their constant value instruction
  // directly.
  auto untyped_inst = resolver.import_insts().GetWithAttachedType(inst_id);
  CARBON_KIND_SWITCH(untyped_inst) {
    case CARBON_KIND(SemIR::AssociatedConstantDecl inst): {
      return TryResolveTypedInst(resolver, inst, const_id);
    }
    case CARBON_KIND(SemIR::ClassDecl inst): {
      return TryResolveTypedInst(resolver, inst, const_id);
    }
    case CARBON_KIND(SemIR::FunctionDecl inst): {
      return TryResolveTypedInst(resolver, inst, const_id);
    }
    case CARBON_KIND(SemIR::ImplDecl inst): {
      return TryResolveTypedInst(resolver, inst, const_id);
    }
    case CARBON_KIND(SemIR::InterfaceDecl inst): {
      return TryResolveTypedInst(resolver, inst, const_id);
    }
    case CARBON_KIND(SemIR::NamedConstraintDecl inst): {
      return TryResolveTypedInst(resolver, inst, const_id);
    }
    default:
      break;
  }

  // Other instructions are imported in a single phase (once their dependencies
  // are all imported).
  CARBON_CHECK(!const_id.has_value());

  auto inst_constant_id = resolver.import_constant_values().Get(inst_id);
  if (!inst_constant_id.is_constant()) {
    // TODO: Import of non-constant BindNames happens when importing `let`
    // declarations.
    CARBON_CHECK(resolver.import_insts().Is<SemIR::AnyBinding>(inst_id),
                 "TryResolveInst on non-constant instruction {0}", inst_id);
    return ResolveResult::Done(SemIR::ConstantId::NotConstant);
  }

  // Import the canonical constant value instruction for `inst_id` directly. We
  // don't try to import the non-canonical `inst_id`.
  auto constant_inst_id =
      resolver.import_constant_values().GetInstId(inst_constant_id);
  CARBON_DCHECK(resolver.import_constant_values().GetConstantInstId(
                    constant_inst_id) == constant_inst_id,
                "Constant value of constant instruction should refer to "
                "the same instruction");

  if (SemIR::IsSingletonInstId(constant_inst_id)) {
    // Constants for builtins can be directly copied.
    return ResolveResult::Done(
        resolver.local_constant_values().Get(constant_inst_id));
  }

  auto untyped_constant_inst =
      resolver.import_insts().GetWithAttachedType(constant_inst_id);
  CARBON_KIND_SWITCH(untyped_constant_inst) {
    case CARBON_KIND(SemIR::AdaptDecl inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::ArrayType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::AssociatedEntity inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::AssociatedEntityType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::BaseDecl inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::AliasBinding inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::SymbolicBinding inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::BoolLiteral inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::BoundMethod inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::Call inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::CharLiteralValue inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::ClassType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::CompleteTypeWitness inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::ConstType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::CppOverloadSetType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::CppOverloadSetValue inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::ExportDecl inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::FacetAccessType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::FacetType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::FacetValue inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::FieldDecl inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::FloatLiteralValue inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::FloatType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::FloatValue inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::FunctionType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::FunctionTypeWithSelfType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::GenericClassType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::GenericInterfaceType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::GenericNamedConstraintType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::LookupImplWitness inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::ImplWitness inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::ImplWitnessAccess inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::ImplWitnessTable inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::ImportRefLoaded inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::IntValue inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::IntType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::MaybeUnformedType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::Namespace inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::OutParamPattern inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::PartialType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::PatternType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::PointerType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::RefBindingPattern inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::RefParamPattern inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::RequireCompleteType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::RequireImplsDecl inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::ReturnSlotPattern inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::SpecificFunction inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::SpecificImplFunction inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::StructAccess inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::StructType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::StructValue inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::SymbolicBindingPattern inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::SymbolicBindingType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::TupleAccess inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::TuplePattern inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::TupleType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::TupleValue inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::UnboundElementType inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::ValueBindingPattern inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::ValueParamPattern inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::VarParamPattern inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::VarPattern inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::VarStorage inst): {
      return TryResolveTypedInst(resolver, inst, constant_inst_id);
    }
    case CARBON_KIND(SemIR::VtableDecl inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    case CARBON_KIND(SemIR::VtablePtr inst): {
      return TryResolveTypedInst(resolver, inst);
    }
    default:
      // Found a canonical instruction which needs to be resolved, but which is
      // not yet handled.
      //
      // TODO: Could we turn this into a compile-time error?
      CARBON_FATAL(
          "Missing case in TryResolveInstCanonical for instruction kind {0}",
          untyped_constant_inst.kind());
  }
}

// Tries to resolve the InstId, returning a constant when ready, or `None` if
// more has been added to the stack. A similar API is followed for all
// following TryResolveTypedInst helper functions.
//
// `const_id` is `None` unless we've tried to resolve this instruction
// before, in which case it's the previous result.
//
// TODO: Error is returned when support is missing, but that should go away.
static auto TryResolveInst(ImportRefResolver& resolver, SemIR::InstId inst_id,
                           SemIR::ConstantId const_id) -> ResolveResult {
  auto inst_const_id = resolver.import_constant_values().GetAttached(inst_id);
  if (!inst_const_id.has_value() || !inst_const_id.is_symbolic()) {
    return TryResolveInstCanonical(resolver, inst_id, const_id);
  }

  // Try to import the generic. This might add new work.
  const auto& symbolic_const =
      resolver.import_constant_values().GetSymbolicConstant(inst_const_id);
  auto generic_const_id =
      GetLocalConstantId(resolver, symbolic_const.generic_id);

  auto inner_const_id = SemIR::ConstantId::None;
  if (const_id.has_value()) {
    // For the third phase, extract the constant value that
    // TryResolveInstCanonical produced previously.
    inner_const_id = resolver.local_constant_values().GetAttached(
        resolver.local_constant_values().GetSymbolicConstant(const_id).inst_id);
  }

  // Import the constant and rebuild the symbolic constant data.
  auto result = TryResolveInstCanonical(resolver, inst_id, inner_const_id);
  if (!result.const_id.has_value()) {
    // First phase: TryResolveInstCanoncial needs a retry.
    return result;
  }

  if (!const_id.has_value()) {
    // Second phase: we have created an unattached constant. Create a
    // corresponding attached constant.
    if (symbolic_const.generic_id.has_value()) {
      result.const_id = resolver.local_constant_values().AddSymbolicConstant(
          {.inst_id =
               resolver.local_constant_values().GetInstId(result.const_id),
           .generic_id = GetLocalGenericId(resolver, generic_const_id),
           .index = symbolic_const.index,
           .dependence = symbolic_const.dependence});
      if (result.decl_id.has_value()) {
        // Overwrite the unattached symbolic constant given initially to the
        // declaration with its final attached symbolic value.
        resolver.local_constant_values().Set(result.decl_id, result.const_id);
      }
    }
  } else {
    // Third phase: perform a consistency check and produce the constant we
    // created in the second phase.
    CARBON_CHECK(result.const_id == inner_const_id,
                 "Constant value changed in third phase.");
    result.const_id = const_id;
  }

  return result;
}

auto ImportRefResolver::ResolveOneInst(SemIR::InstId inst_id)
    -> SemIR::ConstantId {
  work_stack_.push_back({.inst_id = inst_id});
  while (!work_stack_.empty()) {
    auto work = work_stack_.back();
    CARBON_CHECK(work.inst_id.has_value());

    // Step 1: check for a constant value.
    auto existing = FindResolvedConstId(work.inst_id);
    if (existing.const_id.has_value() && !work.retry_with_constant_value) {
      work_stack_.pop_back();
      continue;
    }

    // Step 2: resolve the instruction.
    initial_work_ = work_stack_.size();
    auto [new_const_id, _, retry] =
        TryResolveInst(*this, work.inst_id, existing.const_id);
    CARBON_CHECK(!HasNewWork() || retry);

    CARBON_CHECK(
        !existing.const_id.has_value() || existing.const_id == new_const_id,
        "Constant value changed in third phase.");
    if (!existing.const_id.has_value()) {
      SetResolvedConstId(work.inst_id, existing.indirect_insts, new_const_id);
    }

    // Step 3: pop or retry.
    if (retry) {
      work_stack_[initial_work_ - 1].retry_with_constant_value =
          new_const_id.has_value();
    } else {
      work_stack_.pop_back();
    }
  }
  auto constant_id =
      local_constant_values_for_import_insts().GetAttached(inst_id);
  CARBON_CHECK(constant_id.has_value());
  return constant_id;
}

auto ImportRefResolver::ResolveType(SemIR::TypeId import_type_id)
    -> SemIR::TypeId {
  if (!import_type_id.has_value()) {
    return import_type_id;
  }

  auto import_type_const_id = import_ir().types().GetConstantId(import_type_id);
  CARBON_CHECK(import_type_const_id.has_value());

  if (auto import_type_inst_id = import_ir().types().GetAsTypeInstId(
          import_ir().constant_values().GetInstId(import_type_const_id));
      SemIR::IsSingletonInstId(import_type_inst_id)) {
    // Builtins don't require constant resolution; we can use them directly.
    return GetSingletonType(local_context(), import_type_inst_id);
  } else {
    return local_context().types().GetTypeIdForTypeConstantId(
        ResolveConstant(import_type_id.AsConstantId()));
  }
}

auto ImportRefResolver::GetLocalConstantValueOrPush(SemIR::InstId inst_id)
    -> SemIR::ConstantId {
  if (!inst_id.has_value()) {
    return SemIR::ConstantId::None;
  }
  auto const_id = local_constant_values_for_import_insts().GetAttached(inst_id);
  if (!const_id.has_value()) {
    work_stack_.push_back({.inst_id = inst_id});
  }
  return const_id;
}

auto ImportRefResolver::FindResolvedConstId(SemIR::InstId inst_id)
    -> ResolvedConstId {
  ResolvedConstId result;

  if (auto existing_const_id =
          local_constant_values_for_import_insts().GetAttached(inst_id);
      existing_const_id.has_value()) {
    result.const_id = existing_const_id;
    return result;
  }

  const auto* cursor_ir = &import_ir();
  auto cursor_inst_id = inst_id;

  while (true) {
    auto import_ir_inst_id = cursor_ir->insts().GetImportSource(cursor_inst_id);
    if (!import_ir_inst_id.has_value()) {
      return result;
    }
    auto ir_inst = cursor_ir->import_ir_insts().Get(import_ir_inst_id);
    if (ir_inst.ir_id() == SemIR::ImportIRId::Cpp) {
      local_context().TODO(SemIR::LocId::None,
                           "Unsupported: Importing C++ indirectly");
      SetResolvedConstId(inst_id, result.indirect_insts,
                         SemIR::ErrorInst::ConstantId);
      result.const_id = SemIR::ErrorInst::ConstantId;
      result.indirect_insts.clear();
      return result;
    }

    const auto* prev_ir = cursor_ir;
    auto prev_inst_id = cursor_inst_id;

    cursor_ir = cursor_ir->import_irs().Get(ir_inst.ir_id()).sem_ir;
    auto cursor_ir_id =
        AddImportIR(local_context(), {.decl_id = SemIR::InstId::None,
                                      .is_export = false,
                                      .sem_ir = cursor_ir});
    cursor_inst_id = ir_inst.inst_id();

    CARBON_CHECK(cursor_ir != prev_ir || cursor_inst_id != prev_inst_id, "{0}",
                 cursor_ir->insts().Get(cursor_inst_id));

    if (auto const_id =
            local_context()
                .import_ir_constant_values()
                    [local_context().sem_ir().import_irs().GetRawIndex(
                        cursor_ir_id)]
                .GetAttached(cursor_inst_id);
        const_id.has_value()) {
      SetResolvedConstId(inst_id, result.indirect_insts, const_id);
      result.const_id = const_id;
      result.indirect_insts.clear();
      return result;
    } else {
      result.indirect_insts.push_back(
          SemIR::ImportIRInst(cursor_ir_id, cursor_inst_id));
    }
  }
}

auto ImportRefResolver::SetResolvedConstId(
    SemIR::InstId inst_id, llvm::ArrayRef<SemIR::ImportIRInst> indirect_insts,
    SemIR::ConstantId const_id) -> void {
  local_constant_values_for_import_insts().Set(inst_id, const_id);
  SetIndirectConstantValues(local_context(), indirect_insts, const_id);
}

// Resolves and returns the local contents for an imported instruction block
// of constant instructions.
static auto ResolveLocalInstBlockContents(ImportRefResolver& resolver,
                                          SemIR::InstBlockId import_block_id)
    -> llvm::SmallVector<SemIR::InstId> {
  auto import_block = resolver.import_inst_blocks().Get(import_block_id);

  llvm::SmallVector<SemIR::InstId> inst_ids;
  inst_ids.reserve(import_block.size());
  for (auto import_inst_id : import_block) {
    inst_ids.push_back(resolver.local_constant_values().GetInstId(
        resolver.ResolveOneInst(import_inst_id)));
  }
  return inst_ids;
}

// Resolves and returns a local eval block for a region of an imported
// generic.
static auto ResolveLocalEvalBlock(ImportRefResolver& resolver,
                                  const SemIR::Generic& import_generic,
                                  SemIR::GenericId generic_id,
                                  SemIR::GenericInstIndex::Region region)
    -> SemIR::InstBlockId {
  auto import_block_id = import_generic.GetEvalBlock(region);
  if (!import_block_id.has_value()) {
    return SemIR::InstBlockId::None;
  }

  auto inst_ids = ResolveLocalInstBlockContents(resolver, import_block_id);
  auto eval_block_id = RebuildGenericEvalBlock(resolver.local_context(),
                                               generic_id, region, inst_ids);

  // Set the locations of the instructions in the inst block to match those of
  // the imported instructions.
  for (auto [import_inst_id, local_inst_id] :
       llvm::zip_equal(resolver.import_inst_blocks().Get(import_block_id),
                       resolver.local_inst_blocks().Get(eval_block_id))) {
    auto import_ir_inst_id = AddImportIRInst(resolver, import_inst_id);
    resolver.local_insts().SetLocId(local_inst_id, import_ir_inst_id);
  }
  return eval_block_id;
}

// Fills in the remaining information in a partially-imported generic.
static auto FinishPendingGeneric(ImportRefResolver& resolver,
                                 ImportContext::PendingGeneric pending)
    -> void {
  const auto& import_generic =
      resolver.import_generics().Get(pending.import_id);
  auto& local_generic = resolver.local_generics().Get(pending.local_id);

  local_generic.self_specific_id =
      MakeSelfSpecific(resolver.local_context(),
                       SemIR::LocId(local_generic.decl_id), pending.local_id);
  resolver.AddPendingSpecific({.import_id = import_generic.self_specific_id,
                               .local_id = local_generic.self_specific_id});

  local_generic.definition_block_id =
      ResolveLocalEvalBlock(resolver, import_generic, pending.local_id,
                            SemIR::GenericInstIndex::Region::Definition);
}

// Resolves and returns a local inst block of constant instructions
// corresponding to an imported inst block.
static auto ResolveLocalInstBlock(ImportRefResolver& resolver,
                                  SemIR::InstBlockId import_block_id)
    -> SemIR::InstBlockId {
  if (!import_block_id.has_value()) {
    return SemIR::InstBlockId::None;
  }

  auto inst_ids = ResolveLocalInstBlockContents(resolver, import_block_id);
  return resolver.local_inst_blocks().Add(inst_ids);
}

// Fills in the remaining information in a partially-imported specific.
static auto FinishPendingSpecific(ImportRefResolver& resolver,
                                  ImportContext::PendingSpecific pending)
    -> void {
  const auto& import_specific =
      resolver.import_specifics().Get(pending.import_id);
  auto& local_specific = resolver.local_specifics().Get(pending.local_id);

  if (!local_specific.decl_block_id.has_value()) {
    local_specific.decl_block_id =
        ResolveLocalInstBlock(resolver, import_specific.decl_block_id);
  }

  if (!local_specific.definition_block_id.has_value() &&
      import_specific.definition_block_id.has_value()) {
    local_specific.definition_block_id =
        ResolveLocalInstBlock(resolver, import_specific.definition_block_id);
  }
}

// Perform any work that we deferred until the end of the main Resolve loop.
auto ImportRefResolver::PerformPendingWork() -> void {
  // Note that the individual Finish steps can add new pending work, so keep
  // going until we have no more work to do.
  while (!pending_generics().empty() || !pending_specifics().empty()) {
    // Process generics in the order that we added them because a later
    // generic might refer to an earlier one, and the calls to
    // RebuildGenericEvalBlock assume that the reachable SemIR is in a valid
    // state.
    // TODO: Import the generic eval block rather than calling
    // RebuildGenericEvalBlock to rebuild it so that order doesn't matter.
    for (auto generic_id : GrowingRange(pending_generics())) {
      FinishPendingGeneric(*this, generic_id);
    }
    pending_generics().clear();

    while (!pending_specifics().empty()) {
      FinishPendingSpecific(*this, pending_specifics().pop_back_val());
    }
  }
}

// Returns a list of ImportIRInsts equivalent to the ImportRef currently being
// loaded (including the one pointed at directly by the ImportRef), and the
// final instruction's type ID.
//
// This addresses cases where an ImportRefUnloaded may point at another
// ImportRefUnloaded. The ImportRefResolver requires a SemIR with a
// constant-evaluated version of the instruction to work with.
static auto GetInstForLoad(Context& context,
                           SemIR::ImportIRInstId import_ir_inst_id)
    -> std::pair<llvm::SmallVector<SemIR::ImportIRInst>, SemIR::TypeId> {
  std::pair<llvm::SmallVector<SemIR::ImportIRInst>, SemIR::TypeId> result = {
      {}, SemIR::TypeId::None};
  auto& [import_ir_insts, type_id] = result;

  auto import_ir_inst = context.import_ir_insts().Get(import_ir_inst_id);
  // The first ImportIRInst is added directly because the IR doesn't need to be
  // localized.
  import_ir_insts.push_back(import_ir_inst);
  const auto* cursor_ir =
      context.import_irs().Get(import_ir_inst.ir_id()).sem_ir;

  while (true) {
    auto cursor_inst =
        cursor_ir->insts().GetWithAttachedType(import_ir_inst.inst_id());

    auto import_ref = cursor_inst.TryAs<SemIR::ImportRefUnloaded>();
    if (!import_ref) {
      type_id = cursor_inst.type_id();
      return result;
    }

    import_ir_inst =
        cursor_ir->import_ir_insts().Get(import_ref->import_ir_inst_id);
    cursor_ir = cursor_ir->import_irs().Get(import_ir_inst.ir_id()).sem_ir;
    import_ir_insts.push_back(SemIR::ImportIRInst(
        AddImportIR(context, {.decl_id = SemIR::InstId::None,
                              .is_export = false,
                              .sem_ir = cursor_ir}),
        import_ir_inst.inst_id()));
  }
}

auto LoadImportRef(Context& context, SemIR::InstId inst_id) -> void {
  auto inst = context.insts().TryGetAs<SemIR::ImportRefUnloaded>(inst_id);
  if (!inst) {
    return;
  }

  auto [indirect_insts, load_type_id] =
      GetInstForLoad(context, inst->import_ir_inst_id);

  // The last indirect instruction is the one to resolve. Pop it here because
  // Resolve will assign the constant.
  auto load_ir_inst = indirect_insts.pop_back_val();
  ImportRefResolver resolver(&context, load_ir_inst.ir_id());

  // Loading an import ref creates local constants from the import ones, but
  // shouldn't be generating novel instructions in the semir as a side effect of
  // that process. Doing so in a generic context would also cause them to end up
  // in the eval block, which would be doubly wrong.
  context.inst_block_stack().Push();

  auto type_id = resolver.ResolveType(load_type_id);
  auto constant_id = resolver.Resolve(load_ir_inst.inst_id());

  CARBON_CHECK(
      context.inst_block_stack().PeekCurrentBlockContents().empty(),
      "Importing an instruction shouldn't add new instructions to the "
      "local inst block. Found {0} new instructions, first is {1}: {2}.",
      context.inst_block_stack().PeekCurrentBlockContents().size(),
      context.inst_block_stack().PeekCurrentBlockContents().front(),
      context.insts().Get(
          context.inst_block_stack().PeekCurrentBlockContents().front()));
  context.inst_block_stack().PopAndDiscard();

  // Replace the ImportRefUnloaded instruction with ImportRefLoaded. This
  // doesn't use ReplacePlaceholderImportedInst because it would trigger
  // TryEvalInst, which we want to avoid with ImportRefs.
  context.sem_ir().insts().Set(
      inst_id,
      SemIR::ImportRefLoaded{.type_id = type_id,
                             .import_ir_inst_id = inst->import_ir_inst_id,
                             .entity_name_id = inst->entity_name_id});

  // Store the constant for both the ImportRefLoaded and indirect instructions.
  context.constant_values().Set(inst_id, constant_id);
  SetIndirectConstantValues(context, indirect_insts, constant_id);
}

auto ImportImplsFromApiFile(Context& context) -> void {
  SemIR::ImportIRId import_ir_id = SemIR::ImportIRId::ApiForImpl;
  auto& import_ir = context.import_irs().Get(import_ir_id);
  if (!import_ir.sem_ir) {
    return;
  }

  for (auto [impl_id, _] : import_ir.sem_ir->impls().enumerate()) {
    // Resolve the imported impl to a local impl ID.
    ImportImpl(context, import_ir_id, impl_id);
  }
}

auto ImportImpl(Context& context, SemIR::ImportIRId import_ir_id,
                SemIR::ImplId impl_id) -> void {
  ImportRefResolver resolver(&context, import_ir_id);
  resolver.Resolve(context.import_irs()
                       .Get(import_ir_id)
                       .sem_ir->impls()
                       .Get(impl_id)
                       .first_decl_id());
}

auto ImportInterface(Context& context, SemIR::ImportIRId import_ir_id,
                     SemIR::InterfaceId interface_id) -> SemIR::InterfaceId {
  ImportRefResolver resolver(&context, import_ir_id);
  auto local_id = resolver.Resolve(context.import_irs()
                                       .Get(import_ir_id)
                                       .sem_ir->interfaces()
                                       .Get(interface_id)
                                       .first_decl_id());
  auto local_inst =
      context.insts().Get(context.constant_values().GetInstId(local_id));

  // A non-generic interface will import as a facet type for that single
  // interface.
  if (auto facet_type = local_inst.TryAs<SemIR::FacetType>()) {
    auto single = context.facet_types()
                      .Get(facet_type->facet_type_id)
                      .TryAsSingleExtend();
    CARBON_CHECK(single,
                 "Importing an interface didn't produce a single interface");
    return std::get<SemIR::SpecificInterface>(*single).interface_id;
  }

  // A generic interface will import as a constant of generic interface type.
  auto generic_interface_type =
      context.types().GetAs<SemIR::GenericInterfaceType>(local_inst.type_id());
  return generic_interface_type.interface_id;
}

}  // namespace Carbon::Check