[Clang] C11 `_Generic` matches anything with `void` in trunk build

[fuhsnn]

The static asserts below fail with GCC and Clang <= 20, but pass in Godbolt trunk and clang-21 from Debian nightly repo

_Static_assert(1 == _Generic((int)0 , void : 1),"");
_Static_assert(1 == _Generic((float)0 , void : 1),"");
_Static_assert(1 == _Generic((void*)0 , void : 1),"");
_Static_assert(1 == _Generic((int[4]){}, void : 1),"");

_Static_assert(4 == _Generic(42, void: 4, int: 2),"");
_Static_assert(2 == _Generic(42, int: 2, void: 4),"");

_Static_assert(4 == _Generic(42, void: 4),"");
_Static_assert(2 == _Generic(42, int: 2),"");

Godbolt: https://godbolt.org/z/he9xYrPxq

[frederick-vs-ja]

CC @AaronBallman.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Hsiang-Ying Fu (fuhsnn)

The static asserts below fail with GCC and Clang <= 20, but pass in Godbolt trunk and clang-21 from Debian nightly repo ``` _Static_assert(1 == _Generic((int)0 , void : 1),""); _Static_assert(1 == _Generic((float)0 , void : 1),""); _Static_assert(1 == _Generic((void*)0 , void : 1),""); _Static_assert(1 == _Generic((int[4]){}, void : 1),"");

_Static_assert(4 == _Generic(42, void: 4, int: 2),"");
_Static_assert(2 == _Generic(42, int: 2, void: 4),"");

_Static_assert(4 == _Generic(42, void: 4),"");
_Static_assert(2 == _Generic(42, int: 2),"");

Godbolt: https://godbolt.org/z/he9xYrPxq
</details>

[AaronBallman]

The underlying cause is that we're treating an extension diagnostic as though it were an error here:

llvm-project/clang/lib/Sema/SemaExpr.cpp

Line 1875 in 29db305

if (D != 0) {

So we emit a non-error diagnostic (which is off by default), then we fail as though an error happened, and we hit:

llvm-project/clang/lib/Parse/ParseDeclCXX.cpp

Line 955 in 29db305

if (AssertExpr.isInvalid()) {

which causes us to early return before we ever get the chance to emit the diagnostic for the failed static assertion.

I have a fix in progress.