[clang] Enums differing in underlying type are compatible

[bolshakov-a]

The following code is accepted by clang but rejected by GCC (C23 mode):

enum E : int {A, B, C};

enum E e;

int main() {
  extern enum E Fn();
  enum E {A, B, C};
  extern enum E Fn();
  return __builtin_types_compatible_p(enum E, typeof(e));
}

https://godbolt.org/z/Eab5nz3q9
Compiled by clang, it returns 1, whereas if compiled by GCC (provided that the second Fn() declaration is removed) it returns 0.

I think GCC behavior is correct because 6.2.7 "Compatible type and composite type" states:

For two enumerations, ... if one has a fixed underlying type, then the other shall have a compatible fixed underlying type.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Andrey Ali Khan Bolshakov (bolshakov-a)

The following code is accepted by clang but rejected by GCC (C23 mode): ```cpp enum E : int {A, B, C};

enum E e;

int main() {
extern enum E Fn();
enum E {A, B, C};
extern enum E Fn();
return __builtin_types_compatible_p(enum E, typeof(e));
}

https://godbolt.org/z/Eab5nz3q9
Compiled by clang, it returns 1, whereas if compiled by GCC (provided that the second `Fn()` declaration is removed) it returns 0.

I think GCC behavior is correct because 6.2.7 "Compatible type and composite type" states:
>  For two enumerations, ... if one has a fixed underlying type, then the other shall have a compatible fixed underlying type.

</details>

[shafik]

CC @AaronBallman

[AaronBallman]

Yeah, the AST structural equivalence tester isn't paying attention to fixed underlying type vs unfixed, or for underlying type differences at all. I've got a patch in progress to fix it.

[bolshakov-a]

Btw, there is a disagreement when one of the enumeration types is just opaque-declared: https://godbolt.org/z/hP57YY37v

However, it is unclear to me if it is a Clang bug, a GCC bug, or a core language issue. On the one hand, it is stated in the footnote 46 that an incomplete enum type is not compatible with any other enum type declared in the same translation unit. On the other hand, opaque-declared enumerations are complete (6.7.3.3, p.22).