[libcxx-commits] [PATCH] D68480: Implementation of C++20's P1135R6 for libcxx

Mon Jan 10 07:25:28 PST 2022

ldionne added a comment.

In D68480#3218341 <https://reviews.llvm.org/D68480#3218341>, @mstorsjo wrote:

> In D68480#3218277 <https://reviews.llvm.org/D68480#3218277>, @ldionne wrote:
>
>> I will argue that breaking this ABI on Windows is not a big deal since nobody's vending libc++ on Windows. Or at least if they do, they are completely invisible to us ("us" being the people who develop libc++).
>>
>> Would you be willing to open a patch showing what changes are necessary for each of those? I think we can totally break the ABI on Windows since nobody's shipping it.
>
> I would like to add some nuance to these statements here.
>
> I do ship shared linked libc++ on Windows (as part of the llvm-mingw toolchain distribution), but without any strong ABI guarantees. Ideally ABI wouldn't break between releases, but if it does (especially in a fringe area) it's probably tolerable.
>
> MSYS2 also ships an environment where libc++ is the standard system C++ library, linked shared. That environment is considered somewhat experimental afaik (CC @mati865), so if it breaks some package (which can be fixed by rebuilding), I think they'd consider it tolerable. If none of their packages actually have ended up using and depending on the aspects that may change (I haven't followed closely exactly how widespread it is), the ABI break should be totally transparent though.

Thanks for clarifying, I wasn't aware of that. Like I said, it's incredibly difficult to keep track of everyone who ships libc++, and especially who provides what guarantees to their users.

I added @emaste and @mati865 to the libcxx-vendors "team" here <https://reviews.llvm.org/project/profile/109/> so we can ping you in these situations.

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