std::map::reverse_iterator doesn't work with C++20 when used with incomplete type

I noticed that the use of std::map::reverse_iterator in the below example doesn't work with C++20 but works with C++17 in all compilers.

Demo

Demo MSVC

#include <map>
class C; //incomplete type 

class Something
{
    
    //THIS WORKS IN C++17 as well as C++20 in all compilers
    std::map<int, C>::iterator obj1;
    
    //THIS DOESN'T WORK in C++20 in all compilers but works in C++17 in all compilers
    std::map<int, C>::reverse_iterator  obj2;
};

int main()
{
    Something s;
    return 0;
}

My question is what changed in C++20 so(such) that the use of std::map::reverse_iterator stopped working in all C++20 compilers.