[clang] [C23] Accept an _Atomic underlying type (PR #147802)

[Erich Keane via cfe-commits]

erichkeane wrote:

> avior due to the silent stripping. Given that an atomic type is not the same as its underlying type (in terms of ABI or semantics) I think we should diagnose the behavior with at least a warning. I could even be convinced it should be a warning which defaults to an error because this is just weird.

Yeah, I 100% agree with that.  Warn-as-error is reasonable here IMO (as are ANY qualifiers?, but more so for atomic)

https://github.com/llvm/llvm-project/pull/147802