`<iomanip>`: ADL for the return types of `get_time`/`put_time` looks into the global namespace

Currently, the user-defined overload affects overload resolution. Godbolt link. [ext.manip] arguably disallows this.

#include <iostream>
#include <iomanip>
#include <ctime>

template<class CharT, class Traits, class T>
std::basic_ostream<CharT, Traits>& operator<<(std::basic_ostream<CharT, Traits>& os, T&&) {
    return os << "catch-all\n";
}

int main() {
    std::tm tmv{};
    std::cout << std::put_time(&tmv, "%Y-%M");
}

The reason is that put_time returns _Timeobj<_Elem, const tm*> whose associated namespaces contains the global namespace.

Perhaps the right fix would need to change the return type.

Start by reproducing the Godbolt example and inspect the iomanip implementation of get_time and put_time, along with the linked ext.manip wording. Determine how the return type causes associated-namespace lookup to reach the global namespace. Done means the user-defined catch-all no longer affects overload resolution, with a regression test covering the example.