[C23] Implement WG14 N3037 (#132939)

This changes the type compatibility rules so that it is permitted to
redefine tag types within the same TU so long as they are equivalent
definitions.

It is intentionally not being exposed as an extension in older C
language modes. GCC does not do so and the feature doesn't seem
compelling enough to warrant it.

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -249,6 +249,10 @@ C23 Feature Support
   scope.
 - Fixed a bug where you could not cast a null pointer constant to type
   ``nullptr_t``. Fixes #GH133644.
+- Implemented `WG14 N3037 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3037.pdf>`_
+  which allows tag types to be redefined within the same translation unit so
+  long as both definitions are structurally equivalent (same tag types, same
+  tag names, same tag members, etc).
 - Fixed a failed assertion with an invalid parameter to the ``#embed``
   directive. Fixes #GH126940.
 

clang/include/clang/AST/ASTContext.h

@@ -3147,6 +3147,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
   QualType mergeTransparentUnionType(QualType, QualType,
                                      bool OfBlockPointer=false,
                                      bool Unqualified = false);
+  QualType mergeTagDefinitions(QualType, QualType);
 
   QualType mergeObjCGCQualifiers(QualType, QualType);
 

clang/include/clang/AST/ASTStructuralEquivalence.h

@@ -43,6 +43,9 @@ struct StructuralEquivalenceContext {
   /// key: (from, to, IgnoreTemplateParmDepth)
   using NonEquivalentDeclSet = llvm::DenseSet<std::tuple<Decl *, Decl *, int>>;
 
+  /// The language options to use for making a structural equivalence check.
+  const LangOptions &LangOpts;
+
   /// AST contexts for which we are checking structural equivalence.
   ASTContext &FromCtx, &ToCtx;
 
@@ -76,15 +79,17 @@ struct StructuralEquivalenceContext {
   /// Whether to ignore comparing the depth of template param(TemplateTypeParm)
   bool IgnoreTemplateParmDepth;
 
-  StructuralEquivalenceContext(ASTContext &FromCtx, ASTContext &ToCtx,
+  StructuralEquivalenceContext(const LangOptions &LangOpts, ASTContext &FromCtx,
+                               ASTContext &ToCtx,
                                NonEquivalentDeclSet &NonEquivalentDecls,
                                StructuralEquivalenceKind EqKind,
                                bool StrictTypeSpelling = false,
                                bool Complain = true,
                                bool ErrorOnTagTypeMismatch = false,
                                bool IgnoreTemplateParmDepth = false)
-      : FromCtx(FromCtx), ToCtx(ToCtx), NonEquivalentDecls(NonEquivalentDecls),
-        EqKind(EqKind), StrictTypeSpelling(StrictTypeSpelling),
+      : LangOpts(LangOpts), FromCtx(FromCtx), ToCtx(ToCtx),
+        NonEquivalentDecls(NonEquivalentDecls), EqKind(EqKind),
+        StrictTypeSpelling(StrictTypeSpelling),
         ErrorOnTagTypeMismatch(ErrorOnTagTypeMismatch), Complain(Complain),
         IgnoreTemplateParmDepth(IgnoreTemplateParmDepth) {}
 

clang/include/clang/Basic/DiagnosticASTKinds.td

@@ -478,16 +478,23 @@ def warn_odr_function_type_inconsistent : Warning<
   "external function %0 declared with incompatible types in different "
   "translation units (%1 vs. %2)">,
   InGroup<ODR>;
+def warn_odr_tag_type_with_attributes : Warning<
+  "type %0 has %select{an attribute|a member with an attribute}1 which "
+  "currently causes the types to be treated as though they are incompatible">,
+  InGroup<ODR>, DefaultError;
+def note_odr_attr_here : Note<"attribute %0 here">;
 def err_odr_tag_type_inconsistent
-    : Error<"type %0 has incompatible definitions in different translation "
-            "units">;
+    : Error<"type %0 has incompatible definitions%select{| in different "
+            "translation units}1">;
 def warn_odr_tag_type_inconsistent
-    : Warning<"type %0 has incompatible definitions in different translation "
-              "units">,
+    : Warning<"type %0 has incompatible definitions%select{| in different "
+              "translation units}1">,
       InGroup<ODR>;
 def note_odr_tag_kind_here: Note<
   "%0 is a %select{struct|interface|union|class|enum}1 here">;
 def note_odr_field : Note<"field %0 has type %1 here">;
+def note_odr_field_bit_width : Note<"bit-field %0 has bit-width %1 here">;
+def note_odr_field_not_bit_field : Note<"field %0 is not a bit-field">;
 def note_odr_field_name : Note<"field has name %0 here">;
 def note_odr_missing_field : Note<"no corresponding field here">;
 def note_odr_base : Note<"class has base type %0">;

clang/include/clang/Parse/Parser.h

@@ -2579,7 +2579,8 @@ class Parser : public CodeCompletionHandler {
   void ParseEnumSpecifier(SourceLocation TagLoc, DeclSpec &DS,
                           const ParsedTemplateInfo &TemplateInfo,
                           AccessSpecifier AS, DeclSpecContext DSC);
-  void ParseEnumBody(SourceLocation StartLoc, Decl *TagDecl);
+  void ParseEnumBody(SourceLocation StartLoc, Decl *TagDecl,
+                     SkipBodyInfo *SkipBody = nullptr);
   void ParseStructUnionBody(SourceLocation StartLoc, DeclSpec::TST TagType,
                             RecordDecl *TagDecl);
 

clang/include/clang/Sema/Sema.h

@@ -4346,7 +4346,8 @@ class Sema final : public SemaBase {
   Decl *ActOnEnumConstant(Scope *S, Decl *EnumDecl, Decl *LastEnumConstant,
                           SourceLocation IdLoc, IdentifierInfo *Id,
                           const ParsedAttributesView &Attrs,
-                          SourceLocation EqualLoc, Expr *Val);
+                          SourceLocation EqualLoc, Expr *Val,
+                          SkipBodyInfo *SkipBody = nullptr);
   void ActOnEnumBody(SourceLocation EnumLoc, SourceRange BraceRange,
                      Decl *EnumDecl, ArrayRef<Decl *> Elements, Scope *S,
                      const ParsedAttributesView &Attr);

clang/lib/AST/ASTContext.cpp

@@ -16,6 +16,7 @@
 #include "clang/AST/APValue.h"
 #include "clang/AST/ASTConcept.h"
 #include "clang/AST/ASTMutationListener.h"
+#include "clang/AST/ASTStructuralEquivalence.h"
 #include "clang/AST/ASTTypeTraits.h"
 #include "clang/AST/Attr.h"
 #include "clang/AST/AttrIterator.h"
@@ -11443,6 +11444,22 @@ static QualType mergeEnumWithInteger(ASTContext &Context, const EnumType *ET,
   return {};
 }
 
+QualType ASTContext::mergeTagDefinitions(QualType LHS, QualType RHS) {
+  // C17 and earlier and C++ disallow two tag definitions within the same TU
+  // from being compatible.
+  if (LangOpts.CPlusPlus || !LangOpts.C23)
+    return {};
+
+  // C23, on the other hand, requires the members to be "the same enough", so
+  // we use a structural equivalence check.
+  StructuralEquivalenceContext::NonEquivalentDeclSet NonEquivalentDecls;
+  StructuralEquivalenceContext Ctx(
+      getLangOpts(), *this, *this, NonEquivalentDecls,
+      StructuralEquivalenceKind::Default, /*StrictTypeSpelling=*/false,
+      /*Complain=*/false, /*ErrorOnTagTypeMismatch=*/true);
+  return Ctx.IsEquivalent(LHS, RHS) ? LHS : QualType{};
+}
+
 QualType ASTContext::mergeTypes(QualType LHS, QualType RHS, bool OfBlockPointer,
                                 bool Unqualified, bool BlockReturnType,
                                 bool IsConditionalOperator) {
@@ -11740,7 +11757,7 @@ QualType ASTContext::mergeTypes(QualType LHS, QualType RHS, bool OfBlockPointer,
                               /*AllowCXX=*/false, IsConditionalOperator);
   case Type::Record:
   case Type::Enum:
-    return {};
+    return mergeTagDefinitions(LHS, RHS);
   case Type::Builtin:
     // Only exactly equal builtin types are compatible, which is tested above.
     return {};

clang/lib/AST/ASTImporter.cpp

@@ -2482,8 +2482,9 @@ bool ASTNodeImporter::IsStructuralMatch(Decl *From, Decl *To, bool Complain,
   }
 
   StructuralEquivalenceContext Ctx(
-      Importer.getFromContext(), Importer.getToContext(),
-      Importer.getNonEquivalentDecls(), getStructuralEquivalenceKind(Importer),
+      Importer.getToContext().getLangOpts(), Importer.getFromContext(),
+      Importer.getToContext(), Importer.getNonEquivalentDecls(),
+      getStructuralEquivalenceKind(Importer),
       /*StrictTypeSpelling=*/false, Complain, /*ErrorOnTagTypeMismatch=*/false,
       IgnoreTemplateParmDepth);
   return Ctx.IsEquivalent(From, To);
@@ -4341,7 +4342,8 @@ static bool IsEquivalentFriend(ASTImporter &Importer, FriendDecl *FD1,
 
   ASTImporter::NonEquivalentDeclSet NonEquivalentDecls;
   StructuralEquivalenceContext Ctx(
-      FD1->getASTContext(), FD2->getASTContext(), NonEquivalentDecls,
+      Importer.getToContext().getLangOpts(), FD1->getASTContext(),
+      FD2->getASTContext(), NonEquivalentDecls,
       StructuralEquivalenceKind::Default,
       /* StrictTypeSpelling = */ false, /* Complain = */ false);
   return Ctx.IsEquivalent(FD1, FD2);
@@ -10540,8 +10542,8 @@ bool ASTImporter::IsStructurallyEquivalent(QualType From, QualType To,
     }
   }
 
-  StructuralEquivalenceContext Ctx(FromContext, ToContext, NonEquivalentDecls,
-                                   getStructuralEquivalenceKind(*this), false,
-                                   Complain);
+  StructuralEquivalenceContext Ctx(
+      getToContext().getLangOpts(), FromContext, ToContext, NonEquivalentDecls,
+      getStructuralEquivalenceKind(*this), false, Complain);
   return Ctx.IsEquivalent(From, To);
 }