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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > So we error by default, but still allow users to opt out of the error if they find they need to support an `_Atomic` underlying type.
> 
> Does clang normally reject some valid programs by default with a non-gnu standard selected, intentionally (i.e. not a bug)? 

Yes, though with a warning which defaults to an error so if someone wants their code accepted, they can do so for conformance reasons. As an example: https://godbolt.org/z/MTveG5xs4 We use it more for questionable extensions than we do for standards features though.

> If so, is there an option to turn all of these errors off? It seems like `-w` has no effect, it has to be specifically named `-Wno-underlying-atomic-qualifier-ignored`.

No, the point to defaulting a warning diagnostic as an error is because we want the user to consider carefully what they're trying to do, so you have to consider each category of diagnostic independently.



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