I have tested the code below in Clang, GCC and MSVC compiler.

Code #1:
auto& operator<< ( auto& os, auto&& iter )
    requires requires {
        { os << *iter.begin() } -> same_as<decltype( os )>;
    }
{
    /* ... */
}
Only works on MSVC compiler.

Code #2:
auto& operator<< ( auto& os, auto&& iter )
    requires requires {
        { operator<< ( os, *iter.begin() ) } -> same_as<decltype( os )>;
    }
{
    /* ... */
}
Don't work on all the compilers.

I think it should be defined in the standard, and both behaviors should be
the same.