Don't use `GetCanonicalLocId` when determining what instruction an instruction was imported from. (#5418)

The canonical location of the instruction may be an entirely different
instruction, which the instruction in question was not imported from. In
particular, we shouldn't assume that we can use the constant value of an
instruction that the *location* of an imported instruction refers to as
the constant value of the imported instruction.

The only time we should be looking at the `ImportIRInstId` for a `LocId`
is when determining its location in some other file.

Fixes a crash when importing thunks (which can contain instructions
whose location points to an instruction in a differnt IR).

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/check_unit.cpp

@@ -415,14 +415,11 @@ auto CheckUnit::CheckRequiredDeclarations() -> void {
   for (const auto& function : context_.functions().array_ref()) {
     if (!function.first_owning_decl_id.has_value() &&
         function.extern_library_id == context_.sem_ir().library_id()) {
-      auto function_loc_id =
-          context_.insts().GetCanonicalLocId(function.non_owning_decl_id);
-      CARBON_CHECK(function_loc_id.kind() ==
-                   SemIR::LocId::Kind::ImportIRInstId);
-      auto import_ir_id = context_.sem_ir()
-                              .import_ir_insts()
-                              .Get(function_loc_id.import_ir_inst_id())
-                              .ir_id();
+      auto function_import_id =
+          context_.insts().GetImportSource(function.non_owning_decl_id);
+      CARBON_CHECK(function_import_id.has_value());
+      auto import_ir_id =
+          context_.sem_ir().import_ir_insts().Get(function_import_id).ir_id();
       auto& import_ir = context_.import_irs().Get(import_ir_id);
       if (import_ir.sem_ir->package_id().has_value() !=
           context_.sem_ir().package_id().has_value()) {

toolchain/check/import.cpp

@@ -103,11 +103,16 @@ static auto MakeImportedNamespaceLocIdAndInst(Context& context,
     return SemIR::LocIdAndInst::NoLoc(namespace_inst);
   }
 
+  // If the import was itself imported, use its location.
+  if (auto import_ir_inst_id = context.insts().GetImportSource(import_id);
+      import_ir_inst_id.has_value()) {
+    return MakeImportedLocIdAndInst(context, import_ir_inst_id, namespace_inst);
+  }
+
+  // Otherwise we should have a node location for some kind of namespace
+  // declaration in the current file.
   SemIR::LocId import_loc_id = context.insts().GetCanonicalLocId(import_id);
   switch (import_loc_id.kind()) {
-    case SemIR::LocId::Kind::ImportIRInstId:
-      return MakeImportedLocIdAndInst(
-          context, import_loc_id.import_ir_inst_id(), namespace_inst);
     case SemIR::LocId::Kind::NodeId:
       return SemIR::LocIdAndInst(context.parse_tree().As<Parse::AnyNamespaceId>(
                                      import_loc_id.node_id()),
@@ -115,6 +120,7 @@ static auto MakeImportedNamespaceLocIdAndInst(Context& context,
     case SemIR::LocId::Kind::None:
       // TODO: Either document the use-case for this, or require a location.
       return SemIR::LocIdAndInst::NoLoc(namespace_inst);
+    case SemIR::LocId::Kind::ImportIRInstId:
     case SemIR::LocId::Kind::InstId:
       CARBON_FATAL("Unexpected LocId kind");
   }

toolchain/check/import_ref.cpp

@@ -563,12 +563,12 @@ class ImportRefResolver : public ImportContext {
     auto cursor_inst_id = inst_id;
 
     while (true) {
-      auto loc_id = cursor_ir->insts().GetCanonicalLocId(cursor_inst_id);
-      if (loc_id.kind() != SemIR::LocId::Kind::ImportIRInstId) {
+      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(loc_id.import_ir_inst_id());
+      auto ir_inst = cursor_ir->import_ir_insts().Get(import_ir_inst_id);
 
       const auto* prev_ir = cursor_ir;
       auto prev_inst_id = cursor_inst_id;

toolchain/check/testdata/impl/no_prelude/import_thunk.carbon

@@ -0,0 +1,532 @@
+// 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
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/impl/no_prelude/import_thunk.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/impl/no_prelude/import_thunk.carbon
+
+// --- a.carbon
+
+library "[[@TEST_NAME]]";
+
+interface I {
+  fn F(x: {});
+}
+
+// --- b.carbon
+
+library "[[@TEST_NAME]]";
+import library "a";
+
+class C(X:! ()) {}
+
+impl forall [Y:! ()] C(Y) as I {
+  fn F(x: C(Y)) {}
+}
+
+// --- c.carbon
+
+library "[[@TEST_NAME]]";
+import library "a";
+import library "b";
+
+fn G() {
+  C(()).(I.F)({});
+}
+
+// CHECK:STDOUT: --- a.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %I.type: type = facet_type <@I> [concrete]
+// CHECK:STDOUT:   %Self: %I.type = bind_symbolic_name Self, 0 [symbolic]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %empty_struct_type [concrete]
+// CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
+// CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
+// CHECK:STDOUT:   %I.assoc_type: type = assoc_entity_type @I [concrete]
+// CHECK:STDOUT:   %assoc0: %I.assoc_type = assoc_entity element0, @I.%F.decl [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .I = %I.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %I.decl: type = interface_decl @I [concrete = constants.%I.type] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @I {
+// CHECK:STDOUT:   %Self: %I.type = bind_symbolic_name Self, 0 [symbolic = constants.%Self]
+// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {
+// CHECK:STDOUT:     %x.patt: %pattern_type = binding_pattern x [concrete]
+// CHECK:STDOUT:     %x.param_patt: %pattern_type = value_param_pattern %x.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %x.param: %empty_struct_type = value_param call_param0
+// CHECK:STDOUT:     %.loc5_12.1: type = splice_block %.loc5_12.3 [concrete = constants.%empty_struct_type] {
+// CHECK:STDOUT:       %.loc5_12.2: %empty_struct_type = struct_literal ()
+// CHECK:STDOUT:       %.loc5_12.3: type = converted %.loc5_12.2, constants.%empty_struct_type [concrete = constants.%empty_struct_type]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %x: %empty_struct_type = bind_name x, %x.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %assoc0: %I.assoc_type = assoc_entity element0, %F.decl [concrete = constants.%assoc0]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = %Self
+// CHECK:STDOUT:   .F = %assoc0
+// CHECK:STDOUT:   witness = (%F.decl)
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @F(@I.%Self: %I.type) {
+// CHECK:STDOUT:   fn(%x.param: %empty_struct_type);
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @F(constants.%Self) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- b.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %X: %empty_tuple.type = bind_symbolic_name X, 0 [symbolic]
+// CHECK:STDOUT:   %pattern_type.cb1: type = pattern_type %empty_tuple.type [concrete]
+// CHECK:STDOUT:   %C.type: type = generic_class_type @C [concrete]
+// CHECK:STDOUT:   %C.generic: %C.type = struct_value () [concrete]
+// CHECK:STDOUT:   %C.13320f.1: type = class_type @C, @C(%X) [symbolic]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete]
+// CHECK:STDOUT:   %Y: %empty_tuple.type = bind_symbolic_name Y, 0 [symbolic]
+// CHECK:STDOUT:   %C.13320f.2: type = class_type @C, @C(%Y) [symbolic]
+// CHECK:STDOUT:   %I.type: type = facet_type <@I> [concrete]
+// CHECK:STDOUT:   %Self: %I.type = bind_symbolic_name Self, 0 [symbolic]
+// CHECK:STDOUT:   %F.type.cf0: type = fn_type @F.1 [concrete]
+// CHECK:STDOUT:   %F.bc6: %F.type.cf0 = struct_value () [concrete]
+// CHECK:STDOUT:   %pattern_type.a96: type = pattern_type %empty_struct_type [concrete]
+// CHECK:STDOUT:   %I.impl_witness: <witness> = impl_witness file.%I.impl_witness_table, @impl(%Y) [symbolic]
+// CHECK:STDOUT:   %pattern_type.ccc: type = pattern_type %C.13320f.2 [symbolic]
+// CHECK:STDOUT:   %F.type.0daaa1.1: type = fn_type @F.2, @impl(%Y) [symbolic]
+// CHECK:STDOUT:   %F.49c1ac.1: %F.type.0daaa1.1 = struct_value () [symbolic]
+// CHECK:STDOUT:   %I.facet: %I.type = facet_value %C.13320f.2, (%I.impl_witness) [symbolic]
+// CHECK:STDOUT:   %F.type.0daaa1.2: type = fn_type @F.3, @impl(%Y) [symbolic]
+// CHECK:STDOUT:   %F.49c1ac.2: %F.type.0daaa1.2 = struct_value () [symbolic]
+// CHECK:STDOUT:   %F.specific_fn: <specific function> = specific_function %F.49c1ac.1, @F.2(%Y) [symbolic]
+// CHECK:STDOUT:   %require_complete: <witness> = require_complete_type %C.13320f.2 [symbolic]
+// CHECK:STDOUT:   %C.val: %C.13320f.2 = struct_value () [symbolic]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Main.I: type = import_ref Main//a, I, loaded [concrete = constants.%I.type]
+// CHECK:STDOUT:   %Main.import_ref.e5d = import_ref Main//a, inst17 [no loc], unloaded
+// CHECK:STDOUT:   %Main.import_ref.507 = import_ref Main//a, loc5_14, unloaded
+// CHECK:STDOUT:   %Main.F: %F.type.cf0 = import_ref Main//a, F, loaded [concrete = constants.%F.bc6]
+// CHECK:STDOUT:   %Main.import_ref.5dd: %I.type = import_ref Main//a, inst17 [no loc], loaded [symbolic = constants.%Self]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .I = imports.%Main.I
+// CHECK:STDOUT:     .C = %C.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %default.import = import <none>
+// CHECK:STDOUT:   %C.decl: %C.type = class_decl @C [concrete = constants.%C.generic] {
+// CHECK:STDOUT:     %X.patt: %pattern_type.cb1 = symbolic_binding_pattern X, 0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %.loc5_14.1: type = splice_block %.loc5_14.3 [concrete = constants.%empty_tuple.type] {
+// CHECK:STDOUT:       %.loc5_14.2: %empty_tuple.type = tuple_literal ()
+// CHECK:STDOUT:       %.loc5_14.3: type = converted %.loc5_14.2, constants.%empty_tuple.type [concrete = constants.%empty_tuple.type]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %X.loc5_9.1: %empty_tuple.type = bind_symbolic_name X, 0 [symbolic = %X.loc5_9.2 (constants.%X)]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   impl_decl @impl [concrete] {
+// CHECK:STDOUT:     %Y.patt: %pattern_type.cb1 = symbolic_binding_pattern Y, 0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %C.ref: %C.type = name_ref C, file.%C.decl [concrete = constants.%C.generic]
+// CHECK:STDOUT:     %Y.ref: %empty_tuple.type = name_ref Y, %Y.loc7_14.1 [symbolic = %Y.loc7_14.2 (constants.%Y)]
+// CHECK:STDOUT:     %C.loc7_25.1: type = class_type @C, @C(constants.%Y) [symbolic = %C.loc7_25.2 (constants.%C.13320f.2)]
+// CHECK:STDOUT:     %I.ref: type = name_ref I, imports.%Main.I [concrete = constants.%I.type]
+// CHECK:STDOUT:     %.loc7_19.1: type = splice_block %.loc7_19.3 [concrete = constants.%empty_tuple.type] {
+// CHECK:STDOUT:       %.loc7_19.2: %empty_tuple.type = tuple_literal ()
+// CHECK:STDOUT:       %.loc7_19.3: type = converted %.loc7_19.2, constants.%empty_tuple.type [concrete = constants.%empty_tuple.type]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %Y.loc7_14.1: %empty_tuple.type = bind_symbolic_name Y, 0 [symbolic = %Y.loc7_14.2 (constants.%Y)]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %I.impl_witness_table = impl_witness_table (@impl.%F.decl.loc8_17.2), @impl [concrete]
+// CHECK:STDOUT:   %I.impl_witness: <witness> = impl_witness %I.impl_witness_table, @impl(constants.%Y) [symbolic = @impl.%I.impl_witness (constants.%I.impl_witness)]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @I [from "a.carbon"] {
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = imports.%Main.import_ref.e5d
+// CHECK:STDOUT:   .F = imports.%Main.import_ref.507
+// CHECK:STDOUT:   witness = (imports.%Main.F)
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic impl @impl(%Y.loc7_14.1: %empty_tuple.type) {
+// CHECK:STDOUT:   %Y.loc7_14.2: %empty_tuple.type = bind_symbolic_name Y, 0 [symbolic = %Y.loc7_14.2 (constants.%Y)]
+// CHECK:STDOUT:   %C.loc7_25.2: type = class_type @C, @C(%Y.loc7_14.2) [symbolic = %C.loc7_25.2 (constants.%C.13320f.2)]
+// CHECK:STDOUT:   %I.impl_witness: <witness> = impl_witness file.%I.impl_witness_table, @impl(%Y.loc7_14.2) [symbolic = %I.impl_witness (constants.%I.impl_witness)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %F.type.loc8_17.1: type = fn_type @F.2, @impl(%Y.loc7_14.2) [symbolic = %F.type.loc8_17.1 (constants.%F.type.0daaa1.1)]
+// CHECK:STDOUT:   %F.loc8_17.1: @impl.%F.type.loc8_17.1 (%F.type.0daaa1.1) = struct_value () [symbolic = %F.loc8_17.1 (constants.%F.49c1ac.1)]
+// CHECK:STDOUT:   %F.type.loc8_17.2: type = fn_type @F.3, @impl(%Y.loc7_14.2) [symbolic = %F.type.loc8_17.2 (constants.%F.type.0daaa1.2)]
+// CHECK:STDOUT:   %F.loc8_17.2: @impl.%F.type.loc8_17.2 (%F.type.0daaa1.2) = struct_value () [symbolic = %F.loc8_17.2 (constants.%F.49c1ac.2)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   impl: %C.loc7_25.1 as %I.ref {
+// CHECK:STDOUT:     %F.decl.loc8_17.1: @impl.%F.type.loc8_17.1 (%F.type.0daaa1.1) = fn_decl @F.2 [symbolic = @impl.%F.loc8_17.1 (constants.%F.49c1ac.1)] {
+// CHECK:STDOUT:       %x.patt: @F.2.%pattern_type (%pattern_type.ccc) = binding_pattern x [concrete]
+// CHECK:STDOUT:       %x.param_patt: @F.2.%pattern_type (%pattern_type.ccc) = value_param_pattern %x.patt, call_param0 [concrete]
+// CHECK:STDOUT:     } {
+// CHECK:STDOUT:       %x.param: @F.2.%C.loc8_14.1 (%C.13320f.2) = value_param call_param0
+// CHECK:STDOUT:       %.loc8: type = splice_block %C.loc8_14.2 [symbolic = %C.loc8_14.1 (constants.%C.13320f.2)] {
+// CHECK:STDOUT:         %C.ref: %C.type = name_ref C, file.%C.decl [concrete = constants.%C.generic]
+// CHECK:STDOUT:         %Y.ref: %empty_tuple.type = name_ref Y, @impl.%Y.loc7_14.1 [symbolic = %Y (constants.%Y)]
+// CHECK:STDOUT:         %C.loc8_14.2: type = class_type @C, @C(constants.%Y) [symbolic = %C.loc8_14.1 (constants.%C.13320f.2)]
+// CHECK:STDOUT:       }
+// CHECK:STDOUT:       %x: @F.2.%C.loc8_14.1 (%C.13320f.2) = bind_name x, %x.param
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %F.decl.loc8_17.2: @impl.%F.type.loc8_17.2 (%F.type.0daaa1.2) = fn_decl @F.3 [symbolic = @impl.%F.loc8_17.2 (constants.%F.49c1ac.2)] {
+// CHECK:STDOUT:       %x.patt: %pattern_type.a96 = binding_pattern x [concrete]
+// CHECK:STDOUT:       %x.param_patt: %pattern_type.a96 = value_param_pattern %x.patt, call_param0 [concrete]
+// CHECK:STDOUT:     } {
+// CHECK:STDOUT:       %x.param: %empty_struct_type = value_param call_param0
+// CHECK:STDOUT:       %x: %empty_struct_type = bind_name x, %x.param
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:
+// CHECK:STDOUT:   !members:
+// CHECK:STDOUT:     .C = <poisoned>
+// CHECK:STDOUT:     .Y = <poisoned>
+// CHECK:STDOUT:     .F = %F.decl.loc8_17.1
+// CHECK:STDOUT:     witness = file.%I.impl_witness
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic class @C(%X.loc5_9.1: %empty_tuple.type) {
+// CHECK:STDOUT:   %X.loc5_9.2: %empty_tuple.type = bind_symbolic_name X, 0 [symbolic = %X.loc5_9.2 (constants.%X)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:
+// CHECK:STDOUT:   class {
+// CHECK:STDOUT:     %empty_struct_type: type = struct_type {} [concrete = constants.%empty_struct_type]
+// CHECK:STDOUT:     %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete = constants.%complete_type]
+// CHECK:STDOUT:     complete_type_witness = %complete_type
+// CHECK:STDOUT:
+// CHECK:STDOUT:   !members:
+// CHECK:STDOUT:     .Self = constants.%C.13320f.1
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @F.1(imports.%Main.import_ref.5dd: %I.type) [from "a.carbon"] {
+// CHECK:STDOUT:   fn;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @F.2(@impl.%Y.loc7_14.1: %empty_tuple.type) {
+// CHECK:STDOUT:   %Y: %empty_tuple.type = bind_symbolic_name Y, 0 [symbolic = %Y (constants.%Y)]
+// CHECK:STDOUT:   %C.loc8_14.1: type = class_type @C, @C(%Y) [symbolic = %C.loc8_14.1 (constants.%C.13320f.2)]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %C.loc8_14.1 [symbolic = %pattern_type (constants.%pattern_type.ccc)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %require_complete: <witness> = require_complete_type %C.loc8_14.1 [symbolic = %require_complete (constants.%require_complete)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   fn(%x.param: @F.2.%C.loc8_14.1 (%C.13320f.2)) {
+// CHECK:STDOUT:   !entry:
+// CHECK:STDOUT:     return
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @F.3(@impl.%Y.loc7_14.1: %empty_tuple.type) {
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %Y: %empty_tuple.type = bind_symbolic_name Y, 0 [symbolic = %Y (constants.%Y)]
+// CHECK:STDOUT:   %F.type: type = fn_type @F.2, @impl(%Y) [symbolic = %F.type (constants.%F.type.0daaa1.1)]
+// CHECK:STDOUT:   %F: @F.3.%F.type (%F.type.0daaa1.1) = struct_value () [symbolic = %F (constants.%F.49c1ac.1)]
+// CHECK:STDOUT:   %F.specific_fn.loc8_17.2: <specific function> = specific_function %F, @F.2(%Y) [symbolic = %F.specific_fn.loc8_17.2 (constants.%F.specific_fn)]
+// CHECK:STDOUT:   %C: type = class_type @C, @C(%Y) [symbolic = %C (constants.%C.13320f.2)]
+// CHECK:STDOUT:   %require_complete: <witness> = require_complete_type %C [symbolic = %require_complete (constants.%require_complete)]
+// CHECK:STDOUT:   %C.val: @F.3.%C (%C.13320f.2) = struct_value () [symbolic = %C.val (constants.%C.val)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   fn(%x.param: %empty_struct_type) {
+// CHECK:STDOUT:   !entry:
+// CHECK:STDOUT:     %x.ref: %empty_struct_type = name_ref x, %x.param
+// CHECK:STDOUT:     %F.specific_fn.loc8_17.1: <specific function> = specific_function @impl.%F.decl.loc8_17.1, @F.2(constants.%Y) [symbolic = %F.specific_fn.loc8_17.2 (constants.%F.specific_fn)]
+// CHECK:STDOUT:     %.1: ref @F.3.%C (%C.13320f.2) = temporary_storage
+// CHECK:STDOUT:     %.2: init @F.3.%C (%C.13320f.2) = class_init (), %.1 [symbolic = %C.val (constants.%C.val)]
+// CHECK:STDOUT:     %.3: ref @F.3.%C (%C.13320f.2) = temporary %.1, %.2
+// CHECK:STDOUT:     %.4: ref @F.3.%C (%C.13320f.2) = converted %x.ref, %.3
+// CHECK:STDOUT:     %.5: @F.3.%C (%C.13320f.2) = bind_value %.4
+// CHECK:STDOUT:     %F.call: init %empty_tuple.type = call %F.specific_fn.loc8_17.1(%.5)
+// CHECK:STDOUT:     return
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @C(constants.%X) {
+// CHECK:STDOUT:   %X.loc5_9.2 => constants.%X
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @C(constants.%Y) {
+// CHECK:STDOUT:   %X.loc5_9.2 => constants.%Y
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @impl(constants.%Y) {
+// CHECK:STDOUT:   %Y.loc7_14.2 => constants.%Y
+// CHECK:STDOUT:   %C.loc7_25.2 => constants.%C.13320f.2
+// CHECK:STDOUT:   %I.impl_witness => constants.%I.impl_witness
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %F.type.loc8_17.1 => constants.%F.type.0daaa1.1
+// CHECK:STDOUT:   %F.loc8_17.1 => constants.%F.49c1ac.1
+// CHECK:STDOUT:   %F.type.loc8_17.2 => constants.%F.type.0daaa1.2
+// CHECK:STDOUT:   %F.loc8_17.2 => constants.%F.49c1ac.2
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @F.1(constants.%Self) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @F.2(constants.%Y) {
+// CHECK:STDOUT:   %Y => constants.%Y
+// CHECK:STDOUT:   %C.loc8_14.1 => constants.%C.13320f.2
+// CHECK:STDOUT:   %pattern_type => constants.%pattern_type.ccc
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %require_complete => constants.%require_complete
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @F.1(constants.%I.facet) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @F.3(constants.%Y) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- c.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %G.type: type = fn_type @G [concrete]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %G: %G.type = struct_value () [concrete]
+// CHECK:STDOUT:   %C.type: type = generic_class_type @C [concrete]
+// CHECK:STDOUT:   %C.generic: %C.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete]
+// CHECK:STDOUT:   %X: %empty_tuple.type = bind_symbolic_name X, 0 [symbolic]
+// CHECK:STDOUT:   %empty_tuple: %empty_tuple.type = tuple_value () [concrete]
+// CHECK:STDOUT:   %C.607: type = class_type @C, @C(%empty_tuple) [concrete]
+// CHECK:STDOUT:   %I.type: type = facet_type <@I> [concrete]
+// CHECK:STDOUT:   %Self: %I.type = bind_symbolic_name Self, 0 [symbolic]
+// CHECK:STDOUT:   %I.assoc_type: type = assoc_entity_type @I [concrete]
+// CHECK:STDOUT:   %assoc0: %I.assoc_type = assoc_entity element0, imports.%Main.import_ref.e03 [concrete]
+// CHECK:STDOUT:   %F.type.cf0: type = fn_type @F.1 [concrete]
+// CHECK:STDOUT:   %F.bc6: %F.type.cf0 = struct_value () [concrete]
+// CHECK:STDOUT:   %Y: %empty_tuple.type = bind_symbolic_name Y, 0 [symbolic]
+// CHECK:STDOUT:   %C.13320f.2: type = class_type @C, @C(%Y) [symbolic]
+// CHECK:STDOUT:   %I.impl_witness.7d9: <witness> = impl_witness imports.%I.impl_witness_table, @impl(%Y) [symbolic]
+// CHECK:STDOUT:   %F.type.0daaa1.1: type = fn_type @F.2, @impl(%Y) [symbolic]
+// CHECK:STDOUT:   %F.49c1ac.1: %F.type.0daaa1.1 = struct_value () [symbolic]
+// CHECK:STDOUT:   %pattern_type.ccc: type = pattern_type %C.13320f.2 [symbolic]
+// CHECK:STDOUT:   %F.type.0daaa1.2: type = fn_type @F.3, @impl(%Y) [symbolic]
+// CHECK:STDOUT:   %F.49c1ac.2: %F.type.0daaa1.2 = struct_value () [symbolic]
+// CHECK:STDOUT:   %require_complete: <witness> = require_complete_type %C.13320f.2 [symbolic]
+// CHECK:STDOUT:   %F.specific_fn.8d9: <specific function> = specific_function %F.49c1ac.1, @F.2(%Y) [symbolic]
+// CHECK:STDOUT:   %C.val.56a: %C.13320f.2 = struct_value () [symbolic]
+// CHECK:STDOUT:   %I.impl_witness.02b: <witness> = impl_witness imports.%I.impl_witness_table, @impl(%empty_tuple) [concrete]
+// CHECK:STDOUT:   %F.type.af4856.1: type = fn_type @F.2, @impl(%empty_tuple) [concrete]
+// CHECK:STDOUT:   %F.5fa954.1: %F.type.af4856.1 = struct_value () [concrete]
+// CHECK:STDOUT:   %F.type.af4856.2: type = fn_type @F.3, @impl(%empty_tuple) [concrete]
+// CHECK:STDOUT:   %F.5fa954.2: %F.type.af4856.2 = struct_value () [concrete]
+// CHECK:STDOUT:   %I.facet: %I.type = facet_value %C.607, (%I.impl_witness.02b) [concrete]
+// CHECK:STDOUT:   %.885: type = fn_type_with_self_type %F.type.cf0, %I.facet [concrete]
+// CHECK:STDOUT:   %empty_struct: %empty_struct_type = struct_value () [concrete]
+// CHECK:STDOUT:   %F.specific_fn.4832e8.1: <specific function> = specific_function %F.5fa954.2, @F.3(%empty_tuple) [concrete]
+// CHECK:STDOUT:   %pattern_type.186: type = pattern_type %C.607 [concrete]
+// CHECK:STDOUT:   %F.specific_fn.4832e8.2: <specific function> = specific_function %F.5fa954.1, @F.2(%empty_tuple) [concrete]
+// CHECK:STDOUT:   %C.val.12f: %C.607 = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Main.I: type = import_ref Main//a, I, loaded [concrete = constants.%I.type]
+// CHECK:STDOUT:   %Main.C: %C.type = import_ref Main//b, C, loaded [concrete = constants.%C.generic]
+// CHECK:STDOUT:   %Main.import_ref.eb1c17.1: %empty_tuple.type = import_ref Main//b, loc5_9, loaded [symbolic = @C.%X (constants.%X)]
+// CHECK:STDOUT:   %Main.import_ref.8f2: <witness> = import_ref Main//b, loc5_18, loaded [concrete = constants.%complete_type]
+// CHECK:STDOUT:   %Main.import_ref.572 = import_ref Main//b, inst29 [no loc], unloaded
+// CHECK:STDOUT:   %Main.import_ref.e5d = import_ref Main//a, inst17 [no loc], unloaded
+// CHECK:STDOUT:   %Main.import_ref.c44: %I.assoc_type = import_ref Main//a, loc5_14, loaded [concrete = constants.%assoc0]
+// CHECK:STDOUT:   %Main.F = import_ref Main//a, F, unloaded
+// CHECK:STDOUT:   %Main.import_ref.e03: %F.type.cf0 = import_ref Main//a, loc5_14, loaded [concrete = constants.%F.bc6]
+// CHECK:STDOUT:   %Main.import_ref.5dd: %I.type = import_ref Main//a, inst17 [no loc], loaded [symbolic = constants.%Self]
+// CHECK:STDOUT:   %Main.import_ref.f89: <witness> = import_ref Main//b, loc7_32, loaded [symbolic = @impl.%I.impl_witness (constants.%I.impl_witness.7d9)]
+// CHECK:STDOUT:   %Main.import_ref.eb1c17.2: %empty_tuple.type = import_ref Main//b, loc7_14, loaded [symbolic = @impl.%Y (constants.%Y)]
+// CHECK:STDOUT:   %Main.import_ref.dbf: type = import_ref Main//b, loc7_25, loaded [symbolic = @impl.%C (constants.%C.13320f.2)]
+// CHECK:STDOUT:   %Main.import_ref.f50: type = import_ref Main//b, loc7_30, loaded [concrete = constants.%I.type]
+// CHECK:STDOUT:   %Main.import_ref.047: @impl.%F.type.2 (%F.type.0daaa1.2) = import_ref Main//b, loc8_17, loaded [symbolic = @impl.%F.2 (constants.%F.49c1ac.2)]
+// CHECK:STDOUT:   %I.impl_witness_table = impl_witness_table (%Main.import_ref.047), @impl [concrete]
+// CHECK:STDOUT:   %Main.import_ref.eb1c17.3: %empty_tuple.type = import_ref Main//b, loc7_14, loaded [symbolic = @impl.%Y (constants.%Y)]
+// CHECK:STDOUT:   %Main.import_ref.eb1c17.4: %empty_tuple.type = import_ref Main//b, loc7_14, loaded [symbolic = @impl.%Y (constants.%Y)]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .I = imports.%Main.I
+// CHECK:STDOUT:     .C = imports.%Main.C
+// CHECK:STDOUT:     .G = %G.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %default.import = import <none>
+// CHECK:STDOUT:   %G.decl: %G.type = fn_decl @G [concrete = constants.%G] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @I [from "a.carbon"] {
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = imports.%Main.import_ref.e5d
+// CHECK:STDOUT:   .F = imports.%Main.import_ref.c44
+// CHECK:STDOUT:   witness = (imports.%Main.F)
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic impl @impl(imports.%Main.import_ref.eb1c17.2: %empty_tuple.type) [from "b.carbon"] {
+// CHECK:STDOUT:   %Y: %empty_tuple.type = bind_symbolic_name Y, 0 [symbolic = %Y (constants.%Y)]
+// CHECK:STDOUT:   %C: type = class_type @C, @C(%Y) [symbolic = %C (constants.%C.13320f.2)]
+// CHECK:STDOUT:   %I.impl_witness: <witness> = impl_witness imports.%I.impl_witness_table, @impl(%Y) [symbolic = %I.impl_witness (constants.%I.impl_witness.7d9)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %F.type.1: type = fn_type @F.2, @impl(%Y) [symbolic = %F.type.1 (constants.%F.type.0daaa1.1)]
+// CHECK:STDOUT:   %F.1: @impl.%F.type.1 (%F.type.0daaa1.1) = struct_value () [symbolic = %F.1 (constants.%F.49c1ac.1)]
+// CHECK:STDOUT:   %F.type.2: type = fn_type @F.3, @impl(%Y) [symbolic = %F.type.2 (constants.%F.type.0daaa1.2)]
+// CHECK:STDOUT:   %F.2: @impl.%F.type.2 (%F.type.0daaa1.2) = struct_value () [symbolic = %F.2 (constants.%F.49c1ac.2)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   impl: imports.%Main.import_ref.dbf as imports.%Main.import_ref.f50 {
+// CHECK:STDOUT:   !members:
+// CHECK:STDOUT:     witness = imports.%Main.import_ref.f89
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic class @C(imports.%Main.import_ref.eb1c17.1: %empty_tuple.type) [from "b.carbon"] {
+// CHECK:STDOUT:   %X: %empty_tuple.type = bind_symbolic_name X, 0 [symbolic = %X (constants.%X)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:
+// CHECK:STDOUT:   class {
+// CHECK:STDOUT:     complete_type_witness = imports.%Main.import_ref.8f2
+// CHECK:STDOUT:
+// CHECK:STDOUT:   !members:
+// CHECK:STDOUT:     .Self = imports.%Main.import_ref.572
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @G() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %C.ref: %C.type = name_ref C, imports.%Main.C [concrete = constants.%C.generic]
+// CHECK:STDOUT:   %.loc7_6: %empty_tuple.type = tuple_literal ()
+// CHECK:STDOUT:   %empty_tuple: %empty_tuple.type = tuple_value () [concrete = constants.%empty_tuple]
+// CHECK:STDOUT:   %.loc7_7: %empty_tuple.type = converted %.loc7_6, %empty_tuple [concrete = constants.%empty_tuple]
+// CHECK:STDOUT:   %C: type = class_type @C, @C(constants.%empty_tuple) [concrete = constants.%C.607]
+// CHECK:STDOUT:   %I.ref: type = name_ref I, imports.%Main.I [concrete = constants.%I.type]
+// CHECK:STDOUT:   %F.ref: %I.assoc_type = name_ref F, imports.%Main.import_ref.c44 [concrete = constants.%assoc0]
+// CHECK:STDOUT:   %I.facet: %I.type = facet_value constants.%C.607, (constants.%I.impl_witness.02b) [concrete = constants.%I.facet]
+// CHECK:STDOUT:   %.loc7_8: %I.type = converted %C, %I.facet [concrete = constants.%I.facet]
+// CHECK:STDOUT:   %impl.elem0: %.885 = impl_witness_access constants.%I.impl_witness.02b, element0 [concrete = constants.%F.5fa954.2]
+// CHECK:STDOUT:   %.loc7_16.1: %empty_struct_type = struct_literal ()
+// CHECK:STDOUT:   %empty_struct.loc7_16.1: %empty_struct_type = struct_value () [concrete = constants.%empty_struct]
+// CHECK:STDOUT:   %.loc7_17: %empty_struct_type = converted %.loc7_16.1, %empty_struct.loc7_16.1 [concrete = constants.%empty_struct]
+// CHECK:STDOUT:   %specific_fn: <specific function> = specific_function %impl.elem0, @F.3(constants.%empty_tuple) [concrete = constants.%F.specific_fn.4832e8.1]
+// CHECK:STDOUT:   %empty_struct.loc7_16.2: %empty_struct_type = struct_value () [concrete = constants.%empty_struct]
+// CHECK:STDOUT:   %.loc7_16.2: %empty_struct_type = converted %.loc7_16.1, %empty_struct.loc7_16.2 [concrete = constants.%empty_struct]
+// CHECK:STDOUT:   %F.call: init %empty_tuple.type = call %specific_fn(%.loc7_16.2)
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @F.1(imports.%Main.import_ref.5dd: %I.type) [from "a.carbon"] {
+// CHECK:STDOUT:   fn;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @F.2(imports.%Main.import_ref.eb1c17.3: %empty_tuple.type) [from "b.carbon"] {
+// CHECK:STDOUT:   %Y: %empty_tuple.type = bind_symbolic_name Y, 0 [symbolic = %Y (constants.%Y)]
+// CHECK:STDOUT:   %C: type = class_type @C, @C(%Y) [symbolic = %C (constants.%C.13320f.2)]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %C [symbolic = %pattern_type (constants.%pattern_type.ccc)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %require_complete: <witness> = require_complete_type %C [symbolic = %require_complete (constants.%require_complete)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   fn;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @F.3(imports.%Main.import_ref.eb1c17.4: %empty_tuple.type) [from "b.carbon"] {
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %Y: %empty_tuple.type = bind_symbolic_name Y, 0 [symbolic = %Y (constants.%Y)]
+// CHECK:STDOUT:   %F.type: type = fn_type @F.2, @impl(%Y) [symbolic = %F.type (constants.%F.type.0daaa1.1)]
+// CHECK:STDOUT:   %F: @F.3.%F.type (%F.type.0daaa1.1) = struct_value () [symbolic = %F (constants.%F.49c1ac.1)]
+// CHECK:STDOUT:   %F.specific_fn: <specific function> = specific_function %F, @F.2(%Y) [symbolic = %F.specific_fn (constants.%F.specific_fn.8d9)]
+// CHECK:STDOUT:   %C: type = class_type @C, @C(%Y) [symbolic = %C (constants.%C.13320f.2)]
+// CHECK:STDOUT:   %require_complete: <witness> = require_complete_type %C [symbolic = %require_complete (constants.%require_complete)]
+// CHECK:STDOUT:   %C.val: @F.3.%C (%C.13320f.2) = struct_value () [symbolic = %C.val (constants.%C.val.56a)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   fn;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @C(constants.%X) {
+// CHECK:STDOUT:   %X => constants.%X
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @C(constants.%empty_tuple) {
+// CHECK:STDOUT:   %X => constants.%empty_tuple
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @F.1(constants.%Self) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @C(constants.%Y) {
+// CHECK:STDOUT:   %X => constants.%Y
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @impl(constants.%Y) {
+// CHECK:STDOUT:   %Y => constants.%Y
+// CHECK:STDOUT:   %C => constants.%C.13320f.2
+// CHECK:STDOUT:   %I.impl_witness => constants.%I.impl_witness.7d9
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %F.type.1 => constants.%F.type.0daaa1.1
+// CHECK:STDOUT:   %F.1 => constants.%F.49c1ac.1
+// CHECK:STDOUT:   %F.type.2 => constants.%F.type.0daaa1.2
+// CHECK:STDOUT:   %F.2 => constants.%F.49c1ac.2
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @F.2(constants.%Y) {
+// CHECK:STDOUT:   %Y => constants.%Y
+// CHECK:STDOUT:   %C => constants.%C.13320f.2
+// CHECK:STDOUT:   %pattern_type => constants.%pattern_type.ccc
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %require_complete => constants.%require_complete
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @F.3(constants.%Y) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @impl(constants.%empty_tuple) {
+// CHECK:STDOUT:   %Y => constants.%empty_tuple
+// CHECK:STDOUT:   %C => constants.%C.607
+// CHECK:STDOUT:   %I.impl_witness => constants.%I.impl_witness.02b
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %F.type.1 => constants.%F.type.af4856.1
+// CHECK:STDOUT:   %F.1 => constants.%F.5fa954.1
+// CHECK:STDOUT:   %F.type.2 => constants.%F.type.af4856.2
+// CHECK:STDOUT:   %F.2 => constants.%F.5fa954.2
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @F.3(constants.%empty_tuple) {
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %Y => constants.%empty_tuple
+// CHECK:STDOUT:   %F.type => constants.%F.type.af4856.1
+// CHECK:STDOUT:   %F => constants.%F.5fa954.1
+// CHECK:STDOUT:   %F.specific_fn => constants.%F.specific_fn.4832e8.2
+// CHECK:STDOUT:   %C => constants.%C.607
+// CHECK:STDOUT:   %require_complete => constants.%complete_type
+// CHECK:STDOUT:   %C.val => constants.%C.val.12f
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @F.2(constants.%empty_tuple) {
+// CHECK:STDOUT:   %Y => constants.%empty_tuple
+// CHECK:STDOUT:   %C => constants.%C.607
+// CHECK:STDOUT:   %pattern_type => constants.%pattern_type.186
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %require_complete => constants.%complete_type
+// CHECK:STDOUT: }
+// CHECK:STDOUT:

toolchain/sem_ir/formatter.h

@@ -384,10 +384,11 @@ auto Formatter::FormatEntityStart(llvm::StringRef entity_kind,
   // If this entity was imported from a different IR, annotate the name of
   // that IR in the output before the `{` or `;`.
   if (first_owning_decl_id.has_value()) {
-    auto loc_id = sem_ir_->insts().GetCanonicalLocId(first_owning_decl_id);
-    if (loc_id.kind() == LocId::Kind::ImportIRInstId) {
+    auto import_ir_inst_id =
+        sem_ir_->insts().GetImportSource(first_owning_decl_id);
+    if (import_ir_inst_id.has_value()) {
       auto import_ir_id =
-          sem_ir_->import_ir_insts().Get(loc_id.import_ir_inst_id()).ir_id();
+          sem_ir_->import_ir_insts().Get(import_ir_inst_id).ir_id();
       const auto* import_file = sem_ir_->import_irs().Get(import_ir_id).sem_ir;
       pending_imported_from_ = import_file->filename();
     }

toolchain/sem_ir/ids.h

@@ -947,6 +947,9 @@ struct LocId : public IdBase<LocId> {
   }
 
   // Returns the equivalent `ImportIRInstId` when `kind()` matches or is `None`.
+  // Note that the returned `ImportIRInstId` only identifies a location; it is
+  // not correct to interpret it as the instruction from which another
+  // instruction was imported. Use `InstStore::GetImportSource` for that.
   auto import_ir_inst_id() const -> ImportIRInstId {
     if (!has_value()) {
       return ImportIRInstId::None;

toolchain/sem_ir/import_ir.cpp

@@ -26,12 +26,11 @@ auto ImportIRInst::Print(llvm::raw_ostream& out) const -> void {
 auto GetCanonicalFileAndInstId(const File* sem_ir, SemIR::InstId inst_id)
     -> std::pair<const File*, InstId> {
   while (true) {
-    // Step through an instruction with an imported location to the imported
-    // instruction.
-    if (auto loc_id = sem_ir->insts().GetCanonicalLocId(inst_id);
-        loc_id.kind() == SemIR::LocId::Kind::ImportIRInstId) {
-      auto import_ir_inst =
-          sem_ir->import_ir_insts().Get(loc_id.import_ir_inst_id());
+    // Step through an imported instruction to the instruction it was imported
+    // from.
+    if (auto import_ir_inst_id = sem_ir->insts().GetImportSource(inst_id);
+        import_ir_inst_id.has_value()) {
+      auto import_ir_inst = sem_ir->import_ir_insts().Get(import_ir_inst_id);
       sem_ir = sem_ir->import_irs().Get(import_ir_inst.ir_id()).sem_ir;
       inst_id = import_ir_inst.inst_id();
       continue;

toolchain/sem_ir/inst.h

@@ -476,11 +476,7 @@ class InstStore {
   // most operations on LocId.
   auto GetCanonicalLocId(LocId loc_id) const -> LocId {
     while (loc_id.kind() == LocId::Kind::InstId) {
-      auto inst_id = loc_id.inst_id();
-      CARBON_CHECK(inst_id.index >= 0, "{0}", inst_id.index);
-      CARBON_CHECK(static_cast<size_t>(inst_id.index) < loc_ids_.size(),
-                   "{0} {1}", inst_id.index, loc_ids_.size());
-      loc_id = loc_ids_[inst_id.index];
+      loc_id = GetNonCanonicalLocId(loc_id.inst_id());
     }
     return loc_id;
   }
@@ -488,11 +484,17 @@ class InstStore {
   // Gets the resolved LocId for an instruction. InstId can directly construct
   // an unresolved LocId. This skips that step when a resolved LocId is needed.
   auto GetCanonicalLocId(InstId inst_id) const -> LocId {
-    CARBON_CHECK(inst_id.index >= 0, "{0}", inst_id.index);
-    CARBON_CHECK(inst_id.index < (int)loc_ids_.size(), "{0} {1}", inst_id.index,
-                 loc_ids_.size());
-    auto loc_id = loc_ids_[inst_id.index];
-    return GetCanonicalLocId(loc_id);
+    return GetCanonicalLocId(GetNonCanonicalLocId(inst_id));
+  }
+
+  // Returns the instruction that this instruction was imported from, or
+  // ImportIRInstId::None if this instruction was not generated by importing
+  // another instruction.
+  auto GetImportSource(InstId inst_id) const -> ImportIRInstId {
+    auto loc_id = GetNonCanonicalLocId(inst_id);
+    return loc_id.kind() == LocId::Kind::ImportIRInstId
+               ? loc_id.import_ir_inst_id()
+               : ImportIRInstId::None;
   }
 
   // Overwrites a given instruction with a new value.
@@ -530,6 +532,14 @@ class InstStore {
   // Given a symbolic type, get the corresponding unattached type.
   auto GetUnattachedType(TypeId type_id) const -> TypeId;
 
+  // Gets the specified location for an instruction, without performing any
+  // canonicalization.
+  auto GetNonCanonicalLocId(InstId inst_id) const -> LocId {
+    CARBON_CHECK(static_cast<size_t>(inst_id.index) < loc_ids_.size(),
+                 "{0} {1}", inst_id.index, loc_ids_.size());
+    return loc_ids_[inst_id.index];
+  }
+
   File* file_;
   llvm::SmallVector<LocId> loc_ids_;
   ValueStore<InstId> values_;