build: Require libc++-16 or later

[maflcko]

I am not familiar with macOS, but given that #28622 prescribes a minimum of libc++-16 for macOS, doing the same on Linux could make sense?

This is needed for stuff:

• std::ranges::equal, see refactor: Remove Span operator==, Use std::ranges::equal #29071
• std::source_location, see build: Enable -Wunreachable-code #28999 (comment)

I don't think anyone is using -stdlib=libc++ on Linux, except for the CI and OSS-Fuzz?

The minimum required clang version remains at clang++-15.

[DrahtBot]

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

Code Coverage

For detailed information about the code coverage, see the test coverage report.

Reviews

See the guideline for information on the review process.
A summary of reviews will appear here.

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #29071 (refactor: Remove Span operator==, Use std::ranges::equal by maflcko)
• #28687 (C++20 std::views::reverse by stickies-v)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

[maflcko]

Draft for now to gather conceptual feedback first.

[fanquake]

Given #28687 (comment) and https://godbolt.org/z/n4oTE9sTE the libstdc++-10 requirement isn't going to work here? Looks like this really needs to be clang-16.

[maflcko]

So as long as you don't need deferred concepts instantiation, it should be fine.

[theuni]

More missing in v10: #29081 (comment)

[maflcko]

Thanks, clarified that "libstdc++ (any version) requires at least clang-16"

[fanquake]

and OSS-Fuzz?

So won't this break as soon as we have code that requires libstdc++-16, given that OSS-Fuzz is still pinned to 15?

Are there no other projects using (some?) C++20 code on OSS-Fuzz?

[maflcko]

Not sure if there is much value in being overly accurate here (clang-15 and libc++-16 or clang-16 and libstdc++). If someone can install clang-16 or libc++-16, they can probably also install the other as well?

So I'd say to close this and bump to 16 wholesale?

[fanquake]

So I'd say to close this and bump to 16 wholesale?

I think generally, that would be more straightforward, then supporting varying compiler + std lib combinations, and seems possible to do at this point.