1 files changed, 48 insertions, 6 deletions

diff --git a/clang/lib/AST/ASTStructuralEquivalence.cpp b/clang/lib/AST/ASTStructuralEquivalence.cpp
index f113b32..22bb4cb 100644
--- a/clang/lib/AST/ASTStructuralEquivalence.cpp
+++ b/clang/lib/AST/ASTStructuralEquivalence.cpp
@@ -456,7 +456,9 @@ CheckStructurallyEquivalentAttributes(StructuralEquivalenceContext &Context,
                                       const Decl *D1, const Decl *D2,
                                       const Decl *PrimaryDecl = nullptr) {
   // If either declaration has an attribute on it, we treat the declarations
-  // as not being structurally equivalent.
+  // as not being structurally equivalent unless both declarations are implicit
+  // (ones generated by the compiler like __NSConstantString_tag).
+  //
   // FIXME: this should be handled on a case-by-case basis via tablegen in
   // Attr.td. There are multiple cases to consider: one declaration with the
   // attribute, another without it; different attribute syntax|spellings for
@@ -468,7 +470,7 @@ CheckStructurallyEquivalentAttributes(StructuralEquivalenceContext &Context,
     D1Attr = *D1->getAttrs().begin();
   if (D2->hasAttrs())
     D2Attr = *D2->getAttrs().begin();
-  if (D1Attr || D2Attr) {
+  if ((D1Attr || D2Attr) && !D1->isImplicit() && !D2->isImplicit()) {
     const auto *DiagnoseDecl = cast<TypeDecl>(PrimaryDecl ? PrimaryDecl : D2);
     Context.Diag2(DiagnoseDecl->getLocation(),
                   diag::warn_odr_tag_type_with_attributes)
@@ -884,12 +886,10 @@ static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
       // class comparison.
       if (T1->getTypeClass() == Type::Enum) {
         T1 = T1->getAs<EnumType>()->getDecl()->getIntegerType();
-        if (!T2->isBuiltinType() || T1.isNull()) // Sanity check
-          return false;
+        assert(T2->isBuiltinType() && !T1.isNull()); // Sanity check
       } else if (T2->getTypeClass() == Type::Enum) {
         T2 = T2->getAs<EnumType>()->getDecl()->getIntegerType();
-        if (!T1->isBuiltinType() || T2.isNull()) // Sanity check
-          return false;
+        assert(T1->isBuiltinType() && !T2.isNull()); // Sanity check
       }
       TC = Type::Builtin;
     } else
@@ -2093,6 +2093,48 @@ static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
       !CheckStructurallyEquivalentAttributes(Context, D1, D2))
     return false;
 
+  // In C23, if one enumeration has a fixed underlying type, the other shall
+  // have a compatible fixed underlying type (6.2.7).
+  if (Context.LangOpts.C23) {
+    if (D1->isFixed() != D2->isFixed()) {
+      if (Context.Complain) {
+        Context.Diag2(D2->getLocation(),
+                      Context.getApplicableDiagnostic(
+                          diag::err_odr_tag_type_inconsistent))
+            << Context.ToCtx.getTypeDeclType(D2)
+            << (&Context.FromCtx != &Context.ToCtx);
+        Context.Diag1(D1->getLocation(),
+                      D1->isFixed()
+                          ? diag::note_odr_fixed_underlying_type
+                          : diag::note_odr_missing_fixed_underlying_type)
+            << D1;
+        Context.Diag2(D2->getLocation(),
+                      D2->isFixed()
+                          ? diag::note_odr_fixed_underlying_type
+                          : diag::note_odr_missing_fixed_underlying_type)
+            << D2;
+      }
+      return false;
+    }
+    if (D1->isFixed()) {
+      assert(D2->isFixed() && "enums expected to have fixed underlying types");
+      if (!IsStructurallyEquivalent(Context, D1->getIntegerType(),
+                                    D2->getIntegerType())) {
+        if (Context.Complain) {
+          Context.Diag2(D2->getLocation(),
+                        Context.getApplicableDiagnostic(
+                            diag::err_odr_tag_type_inconsistent))
+              << Context.ToCtx.getTypeDeclType(D2)
+              << (&Context.FromCtx != &Context.ToCtx);
+          Context.Diag2(D2->getLocation(),
+                        diag::note_odr_incompatible_fixed_underlying_type)
+              << D2 << D2->getIntegerType() << D1->getIntegerType();
+        }
+        return false;
+      }
+    }
+  }
+
   llvm::SmallVector<const EnumConstantDecl *, 8> D1Enums, D2Enums;
   auto CopyEnumerators =
       [](auto &&Range, llvm::SmallVectorImpl<const EnumConstantDecl *> &Cont) {