Add Clang 20 CI job and clarify Clang support in README

[ednolan]

Clang/libstdc++ won't build currently.

[coveralls]

Pull Request Test Coverage Report for Build 13888214314

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 13886727097:	0.0%
Covered Lines:	454
Relevant Lines:	454

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💛 - Coveralls

[jwakely]

What doesn't build? I know clang++ 19 with libstdc++ fails, because std::bind_back requires __cpp_explicit_this_parameter, which isn't defined until clang 20.

Is it the non-constexpr error for _M_unex being uninitialized?

[ednolan]

I know clang++ 19 with libstdc++ fails, because std::bind_back requires __cpp_explicit_this_parameter, which isn't defined until clang 20.

Yes

Is it the non-constexpr error for _M_unex being uninitialized?

That one shows up in Clang 19 but not Clang 20. The real reason that Clang 20/libstdc++ fails is that it complains that error: function 'forward_like</*...*/>' with deduced return type cannot be used before it is defined, which I believe may already be fixed with libstdc++ trunk.

Sadly, it's all about to be moot, because I'm going to have to drop support for Clang entirely due to llvm/llvm-project#126489.

[jwakely]

I see, thanks. I wanted to make sure there isn't something I should be fixing (or working around) in libstdc++.

which I believe may already be fixed with libstdc++ trunk.

Yes, trunk and the gcc-14 branch, so it will be fixed for 14.3 and 15.1, both due to be released in the next few months. But that's not much help right now.

Especially with the other deduction guide problem.