[Bug c++/122921] New: The value_type of flat_map's iterator should be pair instead of pair

[gulackeg at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122921

            Bug ID: 122921
           Summary: The value_type of flat_map's iterator should be
                    pair<Key, T> instead of pair<const Key, T>
           Product: gcc
           Version: 15.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gulackeg at gmail dot com
  Target Milestone: ---

Test code:

    // https://godbolt.org/z/bvcY3T961

    #include <flat_map>
    #include <ranges>

    using C = std::flat_map<int, int>;

    using V = std::pair<int, int>;
    using V = C::value_type; // OK

    // - ✅ libc++: OK
    // - ❌ libstdc++: error: type alias redefinition with different types
    //   ('std::ranges::range_value_t<C>' (aka 'std::pair<const int, int>')
    //   vs 'C::value_type' (aka 'pair<int, int>'))
    using V = std::ranges::range_value_t<C>;

According to [flat.map.defn][1], `flat_map`'s `value_type` is specified as
`pair<Key, T>`, and this should also be the `value_type` of `flat_map`'s
`iterator` and `const_iterator`.

[1]: https://eel.is/c++draft/flat.map.defn