[BUG] Apple Clang 13 doesn't support `std::copy_constructible` concept used in `cpp2util.h`

[Neil Henderson]

Describe the bug
The following code in cpp2util.h (lines 367-371) doesn't compile in Apple Clang 13:

template <typename T>
    requires (std::copy_constructible<std::remove_cvref_t<T>>)
auto move(T&& t) -> decltype(auto) {
    return std::move(t);
}

To Reproduce
See a recent build on GitHub Actions
Expand the Build section and scroll to the top

source/../include/cpp2util.h:368:20: error: no template named 'copy_constructible' in namespace 'std'; did you mean 'is_copy_constructible'?
    requires (std::copy_constructible<std::remove_cvref_t<T>>)
              ~~~~~^~~~~~~~~~~~~~~~~~
                   is_copy_constructible

Apple Clang 13 is based off LLVM Clang 12.

Repro on Godbolt for Clang 12.

[Neil Henderson]

To be more specific, the problem isn't with the compiler itself but with the version of the standard library that it ships with.

In Xcode, by default Apple Clang links with libc++. And the version of libc++ that ships with Xcode 13 (Apple Clang 13) doesn't have copy_constructible.

Can repro this on Compiler Explorer:

• Works: Clang 12 with libstdc++
• Fails: Clang 12 with libc++ [This is the closest equivalent to Xcode 13]
• Works: Clang 13 with libc++

[Herb Sutter]

Thanks! It worked on the compilers/libs I'm testing with locally so I thought, hey, it's 2024, why not use a concept? Turns out there was an answer to that question! 😆

Changing to is_copy_constructible_v and running regression...