Describe the bug

Currently, the following program doesn't compile with MSVC STL.

#include <memory>
#include <string>

template <class T>
struct small_ator {
    using value_type = T;

    using size_type       = unsigned short; // !!!
    using difference_type = short; // !!!

    small_ator() = default;
    template <class U>
    constexpr small_ator(const small_ator<U>&) noexcept {}

    T* allocate(size_type n) {
        return std::allocator<T>{}.allocate(n);
    }

    void deallocate(T* p, size_type n) {
        return std::allocator<T>{}.deallocate(p, n);
    }

    friend bool operator==(const small_ator&, const small_ator&) = default;
    template <class U>
    friend constexpr bool operator==(const small_ator&, const small_ator<U>&) {
        return true;
    }
};

int main() {
    std::basic_string<char, std::char_traits<char>, small_ator<char>> s = "hello world";
}

In this case, the size_type is changed in integral promotion, so std::max doesn't work.

Command-line test case

https://godbolt.org/z/xdcqab8xj

Expected behavior

This program compiles, with no C4244 warning (or its friend).

STL version

Effectively, all existing versions since open-sourcing.

Additional context

We should also audit cases where

• the size_type or difference_type is narrower than int, or
• on 32-bit platforms, the size_type or difference_type are 64-bit.

And keeps that there're no compile errors or warnings on narrowing, for all containers and container adaptors that can have unusual size_type and difference_type types.

Tasks

Containers:

• basic_string
• deque
• forward_list
• hive
• list
• vector
• map
• multimap
• set
• multiset
• unordered_map
• unordered_multimap
• unordered_set
• unordered_multiset

(array and inplace_vector are unrelated, as their size_type and difference_type must be size_t and ptrdiff_t respectively.)

Container adaptors (need to verify adaption of standard sequence containers):

• priority_queue
• queue
• stack
• flat_map
• flat_multimap
• flat_set
• flat_multiset