Support -std=c++20/gnu++20

[Romain-Geissler-1A]

Bugzilla Link	44939
Resolution	FIXED
Resolved on	Feb 19, 2020 13:44
Version	trunk
OS	All
Blocks	#43900
CC	@efriedma-quic,@zmodem,@RKSimon,@zygoloid

Extended Description

Hi,

C++20 was marked as (technically) done at the Prague meeting last week, so now there is no need to call this standard C++2a, it is officially C++20. Thus clang should accept -std=c++20/gnu++20 which doesn't seem to be the case with the current trunk (at least on Compiler Explorer). gcc has added this very recently with this commit: https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=fb26050409473f5be54465beca114b7e48de43aa

I also think this is a possible candidate for a backport into the soon to be released Clang 10.

Cheers,
Romain

[zygoloid]

Fixed for Clang 11 in llvmorg-11-init-3585-g24ad1215824.

Hans, would you consider taking a patch that (only) adds the -std=c++20 and -std=gnu++20 flags for Clang 10, or is it too late?

[zmodem]

Fixed for Clang 11 in llvmorg-11-init-3585-g24ad1215824.

Hans, would you consider taking a patch that (only) adds the -std=c++20 and
-std=gnu++20 flags for Clang 10, or is it too late?

Sure, it's not too late.

Do you mean the commit above (which seems fine to me), or do you mean you want to do something smaller? In any case, feel free to push to the branch.

[zygoloid]

I was considering something smaller (just adding the flag and nothing else), but it strikes me that we should probably apply all the changes in that patch at the same time. (For example, the -W flag names should change at the same time as the -std= flag, and it seems appropriate for __cplusplus to change at the same time too.)

Pushed to branch as llvmorg-10.0.0-rc2-24-g5175565cf15.