[clang] [C23] More improved type compatibility for enumerations (PR #150946)

[Aaron Ballman via cfe-commits]

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@@ -2071,6 +2071,44 @@ 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);
+        EnumDecl *Has = D1->isFixed() ? D1 : D2;
+        EnumDecl *Missing = D1->isFixed() ? D1 : D2;
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AaronBallman wrote:

Good catch!

https://github.com/llvm/llvm-project/pull/150946