GNU attributes preceding standard attributes are rejected

[compnerd]

The following exemplifies the problem:

__attribute__((__visibility__("default"))) [[nodiscard]] int f();
[[nodiscard]] __attribute__((__visibility__("default"))) int f();

While GCC accepts both, clang will refuse the first one.

This is currently "expected" behaviour: https://github.com/llvm/llvm-project/blob/main/clang/include/clang/Parse/Parser.h#L2774-L2778

[AaronBallman]

I put the "rejects-valid" tag on this because the code is accepted by GCC and we typically want to follow GCC's behavior with their extensions.

[compnerd]

Mostly complete patch at https://reviews.llvm.org/D137979 ... there is one failure that is unclear that remains to be resolved.

[compnerd]

The differential revision fully passes now and should resolve the issue.