[clang] [C++20] [Modules] Convert '-fexperimental-modules-reduced-bmi' to '-fmodules-reduced-bmi' (PR #114382)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov commented:

I think the overall plan is fine, thanks for working on it!

I think the idea of issuing a warning in order to get users to opt-in to a flag is somewhat novel. Have I missed discussion about this approach, versus simply changing the default?

Regardless, for any new warning we add here, we will probably need to add a targeted way to disable that warning, as otherwise users of Werror are likely to complain.

https://github.com/llvm/llvm-project/pull/114382