Stop pushing a fake generic for the duration of check. (#5326)

This fake generic was used for two reasons:

- The declaration name stack assumes that each declaration name is
processed within a generic scope. This is important if the name might
have generic parameters, which are always parsed even for declarations
that disallow them in check.
- Out-of-line redeclarations of generic entities produce instructions
with symbolic constant values in non-generic scopes.

The former case is addressed by pushing a generic each time we start a
declaration name, even if we will reject generic parameters later. The
latter case is worked around for now by not building a symbolic constant
type or value for instructions that appear outside of any generic, and
will be addressed more completely by #5310 and follow-ups.

toolchain/check/check_unit.cpp

@@ -68,10 +68,6 @@ auto CheckUnit::Run() -> void {
   // Add a block for the file.
   context_.inst_block_stack().Push();
 
-  // TODO: Remove this and the pop in `FinishRun` once we properly push and pop
-  // in the right places.
-  context_.generic_region_stack().Push();
-
   InitPackageScopeAndImports();
 
   // Eagerly import the impls declared in the api file to prepare to redeclare
@@ -509,9 +505,6 @@ auto CheckUnit::CheckRequiredDefinitions() -> void {
 }
 
 auto CheckUnit::FinishRun() -> void {
-  // TODO: Remove this once we properly push and pop in the right places.
-  context_.generic_region_stack().Pop();
-
   CheckRequiredDeclarations();
   CheckRequiredDefinitions();
 

toolchain/check/generic_region_stack.cpp

@@ -11,6 +11,21 @@ auto GenericRegionStack::Push() -> void { dependent_insts_stack_.PushArray(); }
 auto GenericRegionStack::Pop() -> void { dependent_insts_stack_.PopArray(); }
 
 auto GenericRegionStack::AddDependentInst(DependentInst inst) -> void {
+  if (dependent_insts_stack_.empty()) {
+    // If we don't have a generic region here, leave the dependent instruction
+    // unattached. This happens for out-of-line redeclarations of members of
+    // dependent scopes:
+    //
+    //   class A(T:! type) {
+    //     fn F();
+    //   }
+    //   // Has generic type and constant value, but no generic region.
+    //   fn A(T:! type).F() {}
+    //
+    // TODO: Use a different instruction kind for out-of-line definitions and
+    // CHECK this doesn't happen.
+    return;
+  }
   dependent_insts_stack_.AppendToTop(inst);
 }
 

toolchain/check/handle_alias.cpp

@@ -3,6 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include "toolchain/check/context.h"
+#include "toolchain/check/generic.h"
 #include "toolchain/check/handle.h"
 #include "toolchain/check/inst.h"
 #include "toolchain/check/modifiers.h"
@@ -15,6 +16,10 @@ namespace Carbon::Check {
 
 auto HandleParseNode(Context& context, Parse::AliasIntroducerId /*node_id*/)
     -> bool {
+  // Aliases can't be generic, but we might have parsed a generic parameter in
+  // their name, so enter a generic scope just in case.
+  StartGenericDecl(context);
+  // Optional modifiers and the name follow.
   context.decl_introducer_state_stack().Push<Lex::TokenKind::Alias>();
   context.decl_name_stack().PushScopeAndStartName();
   return true;
@@ -31,6 +36,8 @@ auto HandleParseNode(Context& context, Parse::AliasId /*node_id*/) -> bool {
   auto name_context = context.decl_name_stack().FinishName(
       PopNameComponentWithoutParams(context, Lex::TokenKind::Alias));
 
+  DiscardGenericDecl(context);
+
   auto introducer =
       context.decl_introducer_state_stack().Pop<Lex::TokenKind::Alias>();
   LimitModifiersOnDecl(context, introducer, KeywordModifierSet::Access);

toolchain/check/handle_choice.cpp

@@ -23,18 +23,18 @@ namespace Carbon::Check {
 
 auto HandleParseNode(Context& context, Parse::ChoiceIntroducerId node_id)
     -> bool {
+  // This choice is potentially generic.
+  StartGenericDecl(context);
   // Create an instruction block to hold the instructions created as part of the
   // choice signature, such as generic parameters.
   context.inst_block_stack().Push();
+  // There's no modifiers on a choice, but this informs how to typecheck any
+  // generic binding pattern.
+  context.decl_introducer_state_stack().Push<Lex::TokenKind::Choice>();
   // Push the bracketing node.
   context.node_stack().Push(node_id);
   // The choice's name follows.
   context.decl_name_stack().PushScopeAndStartName();
-  // There's no modifiers on a choice, but this informs how to typecheck any
-  // generic binding pattern.
-  context.decl_introducer_state_stack().Push<Lex::TokenKind::Choice>();
-  // This choice is potentially generic.
-  StartGenericDefinition(context);
   return true;
 }
 

toolchain/check/handle_class.cpp

@@ -33,6 +33,8 @@ namespace Carbon::Check {
 
 auto HandleParseNode(Context& context, Parse::ClassIntroducerId node_id)
     -> bool {
+  // This class is potentially generic.
+  StartGenericDecl(context);
   // Create an instruction block to hold the instructions created as part of the
   // class signature, such as generic parameters.
   context.inst_block_stack().Push();
@@ -41,8 +43,6 @@ auto HandleParseNode(Context& context, Parse::ClassIntroducerId node_id)
   // Optional modifiers and the name follow.
   context.decl_introducer_state_stack().Push<Lex::TokenKind::Class>();
   context.decl_name_stack().PushScopeAndStartName();
-  // This class is potentially generic.
-  StartGenericDecl(context);
   return true;
 }
 

toolchain/check/handle_export.cpp

@@ -4,6 +4,7 @@
 
 #include "toolchain/check/context.h"
 #include "toolchain/check/decl_name_stack.h"
+#include "toolchain/check/generic.h"
 #include "toolchain/check/handle.h"
 #include "toolchain/check/inst.h"
 #include "toolchain/check/modifiers.h"
@@ -17,6 +18,9 @@ namespace Carbon::Check {
 
 auto HandleParseNode(Context& context, Parse::ExportIntroducerId /*node_id*/)
     -> bool {
+  // Export declarations can't be generic, but we might have parsed a generic
+  // parameter in their name, so enter a generic scope just in case.
+  StartGenericDecl(context);
   context.decl_introducer_state_stack().Push<Lex::TokenKind::Export>();
   // TODO: Probably need to update DeclNameStack to restrict to only namespaces.
   context.decl_name_stack().PushScopeAndStartName();
@@ -26,6 +30,7 @@ auto HandleParseNode(Context& context, Parse::ExportIntroducerId /*node_id*/)
 auto HandleParseNode(Context& context, Parse::ExportDeclId node_id) -> bool {
   auto name_context = context.decl_name_stack().FinishName(
       PopNameComponentWithoutParams(context, Lex::TokenKind::Export));
+  DiscardGenericDecl(context);
   context.decl_name_stack().PopScope();
 
   auto introducer =

toolchain/check/handle_function.cpp

@@ -40,6 +40,8 @@ namespace Carbon::Check {
 
 auto HandleParseNode(Context& context, Parse::FunctionIntroducerId node_id)
     -> bool {
+  // The function is potentially generic.
+  StartGenericDecl(context);
   // Create an instruction block to hold the instructions created as part of the
   // function signature, such as parameter and return types.
   context.inst_block_stack().Push();
@@ -48,8 +50,6 @@ auto HandleParseNode(Context& context, Parse::FunctionIntroducerId node_id)
   // Optional modifiers and the name follow.
   context.decl_introducer_state_stack().Push<Lex::TokenKind::Fn>();
   context.decl_name_stack().PushScopeAndStartName();
-  // The function is potentially generic.
-  StartGenericDecl(context);
   return true;
 }
 

toolchain/check/handle_impl.cpp

@@ -28,6 +28,9 @@ namespace Carbon::Check {
 
 auto HandleParseNode(Context& context, Parse::ImplIntroducerId node_id)
     -> bool {
+  // This might be a generic impl.
+  StartGenericDecl(context);
+
   // Create an instruction block to hold the instructions created for the type
   // and interface.
   context.inst_block_stack().Push();
@@ -42,9 +45,6 @@ auto HandleParseNode(Context& context, Parse::ImplIntroducerId node_id)
   // consistent to imagine that it does. This also gives us a scope for implicit
   // parameters.
   context.decl_name_stack().PushScopeAndStartName();
-
-  // This might be a generic impl.
-  StartGenericDecl(context);
   return true;
 }
 

toolchain/check/handle_interface.cpp

@@ -21,6 +21,8 @@ namespace Carbon::Check {
 
 auto HandleParseNode(Context& context, Parse::InterfaceIntroducerId node_id)
     -> bool {
+  // This interface is potentially generic.
+  StartGenericDecl(context);
   // Create an instruction block to hold the instructions created as part of the
   // interface signature, such as generic parameters.
   context.inst_block_stack().Push();
@@ -29,8 +31,6 @@ auto HandleParseNode(Context& context, Parse::InterfaceIntroducerId node_id)
   // Optional modifiers and the name follow.
   context.decl_introducer_state_stack().Push<Lex::TokenKind::Interface>();
   context.decl_name_stack().PushScopeAndStartName();
-  // This interface is potentially generic.
-  StartGenericDecl(context);
   return true;
 }
 

toolchain/check/handle_namespace.cpp

@@ -4,6 +4,7 @@
 
 #include "toolchain/check/context.h"
 #include "toolchain/check/decl_introducer_state.h"
+#include "toolchain/check/generic.h"
 #include "toolchain/check/handle.h"
 #include "toolchain/check/inst.h"
 #include "toolchain/check/modifiers.h"
@@ -19,6 +20,9 @@ namespace Carbon::Check {
 
 auto HandleParseNode(Context& context, Parse::NamespaceStartId /*node_id*/)
     -> bool {
+  // Namespaces can't be generic, but we might have parsed a generic parameter
+  // in their name, so enter a generic scope just in case.
+  StartGenericDecl(context);
   // Optional modifiers and the name follow.
   context.decl_introducer_state_stack().Push<Lex::TokenKind::Namespace>();
   context.decl_name_stack().PushScopeAndStartName();
@@ -35,6 +39,8 @@ auto HandleParseNode(Context& context, Parse::NamespaceId node_id) -> bool {
   auto name_context = context.decl_name_stack().FinishName(
       PopNameComponentWithoutParams(context, Lex::TokenKind::Namespace));
 
+  DiscardGenericDecl(context);
+
   auto introducer =
       context.decl_introducer_state_stack().Pop<Lex::TokenKind::Namespace>();
   LimitModifiersOnDecl(context, introducer, KeywordModifierSet::None);