[libcxx-commits] [libcxx] Revert "[libc++] Allow the use of extensions in the implementation (#79532)" (PR #85792)

[Sam McCall via libcxx-commits]

sam-mccall wrote:

Thanks!

> I see other solutions being mentioned in #79532 though, should we be exploring those instead?

It seems like there are several options with tradeoffs, and I don't feel informed enough to push for one of them. If someone closer to the project can make the call then absolutely.

If there's still some discussion to be had, I'd be grateful if we could revert back to a good state first.
(We consume libc++ close to head, so we'll have to start carrying a downstream patch for this soon, which is no fun).

@frederick-vs-ja 
> When building modules, it shouldn't be needed to suppress warnings on C++14/17/20 features, right?

Mostly. (You can use Clang modules without C++20, but I think the relatively-few who did that are on C++20 now anyway). You could reduce the suppression pragmas this way (could gate on language version rather than modules-enabled?). But it also feels like a short-term fix to what may simply be the wrong mechanism.

https://github.com/llvm/llvm-project/pull/85792