Does libc++'s span have a non-standard converting constructor?

[ldionne]

I just came across these constructors which I can't find anywhere in the Standard:

llvm-project/libcxx/include/span

Lines 296 to 303 in 4924172

	template <__span_array_convertible<element_type> _OtherElementType>
	_LIBCPP_HIDE_FROM_ABI constexpr span(const span<_OtherElementType, _Extent>& __other) : __data_{__other.data()} {}
	template <__span_array_convertible<element_type> _OtherElementType>
	_LIBCPP_HIDE_FROM_ABI constexpr explicit span(const span<_OtherElementType, dynamic_extent>& __other) noexcept
	: __data_{__other.data()} {
	_LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(_Extent == __other.size(), "size mismatch in span's constructor (other span)");
	}

Are they non-standard constructors we provide?

Also, it is possible that the constructor at line 297 is missing a noexcept (thanks @mclow for the heads up).

[mclow]

This is in the implementation of the fixed-size span.
These constructors two versions of last constructor listed in https://eel.is/c++draft/views#span.cons

Constraints listed here: https://eel.is/c++draft/views#span.cons-22

[ldionne]

@mclow I think you're totally right. PR open.