指向 volatile 的指针的迭代器特征

Question

This code

#include <iterator>
#include <type_traits>

static_assert(std::is_same_v<typename std::iterator_traits<volatile int *>::value_type, volatile int>);

compiles on latest GCC and clang, but fails on MSVC 2019, that seems to remove volatile qualifier. See here on godbolt .

The const is removed, due to the standard specialization of std::iterator_traits for const T* and T* , but I think volatile should be kept.

Who is right?

Edit: I'm compiling with C++17.

Answer 1

Extending the link comment by @康桓瑋 to an answer:

This is LWG issue 2952 . Before its resolution value_type would be volatile -qualified, but its resolution changes it to remove the volatile qualification.

The resolution is incorporated into C++20 and MSVC, GCC and Clang all seem to implement it as such. (Meaning that the static_assert in the question fails when the compiler is set to C++20 mode.)

With regards to whether the resolution should be applied as a defect report to previous revisions of the standard, you can read some discussion here: https://github.com/microsoft/STL/issues/2612 .

It seems that Microsoft's standard library implementation as well as LLVM's libc++ apply the issue resolution also to previous standard revision modes, while GCC's libstdc++ doesn't. I could not find any bug report or similar discussing the latter's choice.