[ Kona 2022-11-12; Set priority to 2 ]

This issue is discovered when implementing the span part of P2278R4 in MSVC STL.

P2278R4 added the const_iterator member type to std::span, which required its iterator type to model std::input_iterator (same for const_reverse_iterator and reverse_iterator).

However, when element_type is volatile T and T is a class type, the iterator type generally fails to satisfy input_iterator, because:

1. input_iterator<iterator> requires indirectly_readable<iterator>;

2. indirectly_readable<iterator> requires common_reference_with<iterator_reference_t<iterator>&&, iterator_rvalue_reference_t<iterator>&&>, that is common_reference_with<volatile T&, volatile T&&>;

3. common_reference_t<volatile T&, volatile T&&> is T (which is problematic), and thus common_reference_with<volatile T&, volatile T&&> requires both convertible_to<volatile T&, T> and convertible_to<volatile T&&, T>;

4. However, the class type T generally doesn't have constructors from volatile T or const volatile T glvalues, and thus neither convertible_to<volatile T&, T> and convertible_to<volatile T&&, T> is satisfied.

Ideally, span should not require any form of construction of element_type. Although usages of class types provided by the standard library via volatile glvalues are generally not supported, I think span<volatile T, E> is still useful for some user-defined class type T, and thus shouldn't be forbidden.