[C23] Handle type compatibility for enumerations better

clang/lib/AST/ASTStructuralEquivalence.cpp

@@ -870,7 +870,29 @@ static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
     else if (T1->getTypeClass() == Type::FunctionNoProto &&
              T2->getTypeClass() == Type::FunctionProto)
       TC = Type::FunctionNoProto;
-    else
+    else if (Context.LangOpts.C23 && !Context.StrictTypeSpelling &&
+             (T1->getTypeClass() == Type::Enum ||
+              T2->getTypeClass() == Type::Enum)) {
+      // In C23, if not being strict about token equivalence, we need to handle
+      // the case where one type is an enumeration and the other type is an
+      // integral type.
+      //
+      // C23 6.7.3.3p16: The enumerated type is compatible with the underlying
+      // type of the enumeration.
+      //
+      // Treat the enumeration as its underlying type and use the builtin type
+      // class comparison.
+      if (T1->getTypeClass() == Type::Enum) {
+        T1 = T1->getAs<EnumType>()->getDecl()->getIntegerType();
+        if (!T2->isBuiltinType() || T1.isNull()) // Sanity check
+          return false;
+      } else if (T2->getTypeClass() == Type::Enum) {
+        T2 = T2->getAs<EnumType>()->getDecl()->getIntegerType();
+        if (!T1->isBuiltinType() || T2.isNull()) // Sanity check
+          return false;
+      }
+      TC = Type::Builtin;
+    } else
       return false;
   }
 

clang/test/C/C23/n3037.c

@@ -401,3 +401,39 @@ _Static_assert(0 == _Generic(inner_anon_tagged.untagged, struct { int i; } : 1,
 // unions and structures are both RecordDecl objects, whereas EnumDecl is not).
 enum { E_Untagged1 } nontag_enum; // both-note {{previous definition is here}}
 _Static_assert(0 == _Generic(nontag_enum, enum { E_Untagged1 } : 1, default : 0)); // both-error {{redefinition of enumerator 'E_Untagged1'}}
+
+// Test that enumerations are compatible with their underlying type, but still
+// diagnose when "same type" is required rather than merely "compatible type".
+enum E1 : int { e1 }; // Fixed underlying type
+enum E2 { e2 };       // Unfixed underlying type, defaults to int or unsigned int
+
+struct GH149965_1 { int h; };
+// This typeof trick is used to get the underlying type of the enumeration in a
+// platform agnostic way.
+struct GH149965_2 { __typeof__(+(enum E2){}) h; };
+void gh149965(void) {
+  extern struct GH149965_1 x1; // c17-note {{previous declaration is here}}
+  extern struct GH149965_2 x2; // c17-note {{previous declaration is here}}
+
+  // Both the structure and the variable declarations are fine because only a
+  // compatible type is required, not the same type, because the structures are
+  // declared in different scopes.
+  struct GH149965_1 { enum E1 h; };
+  struct GH149965_2 { enum E2 h; };
+
+  extern struct GH149965_1 x1; // c17-error {{redeclaration of 'x1'}}
+  extern struct GH149965_2 x2; // c17-error {{redeclaration of 'x2'}}
+
+  // However, in the same scope, the same type is required, not just compatible
+  // types.
+  // FIXME: this should be an error in both C17 and C23 mode.
+  struct GH149965_3 { int h; };     // c17-note {{previous definition is here}}
+  struct GH149965_3 { enum E1 h; }; // c17-error {{redefinition of 'GH149965_3'}}
+
+  // For Clang, the composite type after declaration merging is the enumeration
+  // type rather than an integer type.
+  enum E1 *eptr;
+  [[maybe_unused]] __typeof__(x1.h) *ptr = eptr;
+  enum E2 *eptr2;
+  [[maybe_unused]] __typeof__(x2.h) *ptr2 = eptr2;
+}