[Clang] C23 struct compatiblity rule differs from GCC

[fuhsnn]

The last case shows that clang-21 currently treat two same-record structs without tag as compatible, while GCC doesn't.

struct     {int i;} nontag;
struct tag {int i;} tagged;

static_assert(1 == _Generic(tagged, struct tag {int i;}:1, default:0)); // both 1
static_assert(0 == _Generic(tagged, struct     {int i;}:1, default:0)); // both 0
static_assert(0 == _Generic(nontag, struct tag {int i;}:1, default:0)); // both 0
static_assert(0 == _Generic(nontag, struct     {int i;}:1, default:0)); // gcc:0 clang:1

https://godbolt.org/z/bGWfYT5WE

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Hsiang-Ying Fu (fuhsnn)

The last case shows that clang-21 currently treat two same-record structs without tag as compatible, while GCC doesn't. ``` struct {int i;} nontag; struct tag {int i;} tagged;

static_assert(1 == _Generic(tagged, struct tag {int i;}:1, default:0)); // both 1
static_assert(0 == _Generic(tagged, struct {int i;}:1, default:0)); // both 0
static_assert(0 == _Generic(nontag, struct tag {int i;}:1, default:0)); // both 0
static_assert(0 == _Generic(nontag, struct {int i;}:1, default:0)); // gcc:0 clang:1

https://godbolt.org/z/bGWfYT5WE
</details>

[AaronBallman]

I didn't read the standard hard enough and so I missed that the tag is the identifier for the type, not the keyword and the identifier, and so the two types are only compatible across translation units rather than within the same translation unit because they don't have the same tag (they don't have any tag at all).