[Clang] Clang-cl mode is bug-compatible with MSVC on completeness of enumeration types

[frederick-vs-ja]

The following example should be rejected, because E is an enumeration type without fixed underlying type and thus incomplete within it's enumerator list. However, Clang currently accepts in Clang-cl mode (demo).

enum E {
  n = sizeof(E),
};

Another example shows that the underlying type is assumed to be int here.

As MSVC is (incorrectly) doing the same, perhaps such behavior is kept for bug-compatibility. But given such "extension" can be undesired, because the underlying type can become another type when the enumeration becomes complete, perhaps we should consider turning it off by default, or even removing it one day.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: A. Jiang (frederick-vs-ja)

The following example should be rejected, because `E` is an enumeration type without fixed underlying type and thus incomplete within it's enumerator list. However, Clang currently accepts in Clang-cl mode ([demo](https://godbolt.org/z/8sr87nnj9)).

enum E {
  n = sizeof(E),
};

Another example shows that the underlying type is assumed to be int here.

As MSVC is (incorrectly) doing the same, perhaps such behavior is kept for bug-compatibility. But given such "extension" can be undesired, because the underlying type can become another type when the enumeration becomes complete, perhaps we should consider turning it off by default, or even removing it one day.

[efriedma-quic]

I suspect this isn't intentional. See b0a17ed / #27472 . Might need to downgrade it to an ExtWarn, though.

because the underlying type can become another type when the enumeration becomes complete

Under MSVC ABI, non-fixed enums are always int.

[frederick-vs-ja]

because the underlying type can become another type when the enumeration becomes complete

Under MSVC ABI, non-fixed enums are always int.

This seems to be another bug-compatibity choice.

MSVC has recently fixed this under a new option. See
https://learn.microsoft.com/en-us/cpp/build/reference/zc-enumtypes?view=msvc-170.