[libcxx-commits] [libcxx] [libc++] Default the allocator argument for most string constructors (PR #169901)

[A. Jiang via libcxx-commits]

frederick-vs-ja wrote:

@bgra8 It seems to me that Clang and libc++ are doing right things.

[P4149R1](https://wg21.link/p4149r1) (applied as a Defect Report) clarifies that default arguments are not part of the immediate context, and [[basic.string.general]](https://eel.is/c++draft/basic.string.general) specifies that there should be a default argument. So as discussed in [P4149R1](https://wg21.link/p4149r1), there should be a hard error when detecting the value of `is_constructible_v`.

https://github.com/llvm/llvm-project/pull/169901