http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#879
not implemented, while clang3.6 works.
Code:

#include <cstddef>
struct X_
        {
        operator std::nullptr_t() { return nullptr; }
        };
struct Y_
        {
        operator std::nullptr_t() { return nullptr; }
        };
int main()
{
    X_ x;
    Y_ y;
    bool z = x != y;

    return 0;
}

Compile error:
main.cpp: In function 'int main()':

main.cpp:15:28: error: no match for 'operator!=' (operand types are 'X_' and 'Y_')

                 bool z = x != y;

                            ^