[C23] Implement WG14 N3037

clang/docs/ReleaseNotes.rst

@@ -161,6 +161,10 @@ 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).
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------

clang/include/clang/AST/ASTContext.h

@@ -3130,6 +3130,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
   QualType mergeTransparentUnionType(QualType, QualType,
                                      bool OfBlockPointer=false,
                                      bool Unqualified = false);
+  QualType mergeTagTypes(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

@@ -2555,7 +2555,8 @@ 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

@@ -4078,7 +4078,8 @@ 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"
@@ -11399,6 +11400,22 @@ static QualType mergeEnumWithInteger(ASTContext &Context, const EnumType *ET,
   return {};
 }
 
+QualType ASTContext::mergeTagTypes(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) {
@@ -11695,7 +11712,7 @@ QualType ASTContext::mergeTypes(QualType LHS, QualType RHS, bool OfBlockPointer,
                               /*AllowCXX=*/false, IsConditionalOperator);
   case Type::Record:
   case Type::Enum:
-    return {};
+    return mergeTagTypes(LHS, RHS);
   case Type::Builtin:
     // Only exactly equal builtin types are compatible, which is tested above.
     return {};

clang/lib/AST/ASTImporter.cpp

@@ -2489,8 +2489,9 @@ 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);
@@ -4345,7 +4346,8 @@ 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);
@@ -10563,8 +10565,8 @@ 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);
 }