ldionne added a comment.

In D126907#3746477 <https://reviews.llvm.org/D126907#3746477>, @Mordante wrote:

> Unfortunately there are a lot of different options and combination of options 
> to test libc++.
> So it's indeed not possible to test all options with once check-cxx 
> invocation.

This ^. We could include all the libc++ configurations in a single `check-cxx` 
invokation, but the tests would run for like 70 hours on most machines. 
Instead, we test different configurations in different CI jobs.

In D126907#3777056 <https://reviews.llvm.org/D126907#3777056>, @erichkeane 
wrote:

> Still WIP, but uploading to show that I'm still on this :/
>
> The two modules related issues from libcxx are now fixed (as reported by 
> @Mordante), and that configuration builds and passes all tests with MOST of 
> this change.
>
> HOWEVER, the first of two fixes for @wlei messes up how constraint 
> expressions on class templates are compared, so the result is ~800 additional 
> libcxx failures.  I'm still working through that.

Just to make sure we're on the same page, I assume most of those issues are 
things that would have been encountered in user code otherwise and filed as 
bugs. So in a way, I think libc++ is simply acting like some kind of 
early-in-the-loop pre-release testing. This is similar to what some other 
open-source projects like Chrome do -- they build from trunk often and will 
report actual issues to us early. I do get why it can be annoying and 
disruptive to landing patches sometimes, though.

Concretely, what we could do is probably add `LLVM_ENABLE_RUNTIMES=libcxx` to 
the Clang pre-commit CI that already runs. That would catch some (but of course 
not all) issues. In particular, that should catch issues that would otherwise 
immediately break the libc++ "Bootstrapping build" CI job. For other issues 
(like an arbitrary combination of `-std=c++xy -fmodules -fno-rtti`), I think we 
can only rely on slower feedback when libc++ updates the nightly version of 
Clang that we use in our CI (which would act like a super-early adopter from 
the POV of Clang at that point).


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D126907/new/

https://reviews.llvm.org/D126907

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