std::span with volatile non copyable type cannot be constructed

[apache-hb]

Constructing a span containing a non-copyable type fails if volatile qualified, requires specifying a volatile overload for both copy and move constructors. example. Additionally the behaviour of libc++ and libstdc++ differ when deducing the size of an array type constructor if the type is volatile. example

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Elliot (apache-hb)

Details

Constructing a span containing a non-copyable type fails if volatile qualified, requires specifying a volatile overload for both copy and move constructors. [example](https://godbolt.org/z/zrxz7vT6j). Additionally the behaviour of libc++ and libstdc++ differ when deducing the size of an array type constructor if the type is volatile. [example](https://godbolt.org/z/r6W3KPGW4)

[frederick-vs-ja]

I believe this is a library issue and Clang is doing the right things. But the issue is somehow by design, see LWG3813.

IMO the crux is that const volatile Comparator* doesn't satisfy std::contiguous_iterator (due to the problematic detection for common_reference_t), which is currently mandatory in the standard.

In the latter posted example, GCC (14.2) uses an old version of libstdc++ which was a little more buggy. The constructors were fixed by gcc-mirror/gcc@e95bda0 on the trunk (cherry-picked as gcc-mirror/gcc@83fa082, expected to be available in GCC 14.3).

[philnik777]

Since it's LWG territory we can close it. If LWG decides what to do at any point we'll track that instead.