C++11 operator"" with double parameter

Consider:

struct str {};

str operator"" _X(long double d) {
    return str();
}

This compiles fine with g++ 4.7.2 Wall std=c++11

but now if I give a double :

str operator"" _X(double d) {
    return str();
}

I get the following error message: main.cpp|3|error: 'str operator"" _X(double)' has invalid argument list

What is the problem ? Has this something to do with "It is not possible to redefine the meaning of a built-in literal suffix" (Stroustrup FAQ) ? Can you think of a workaround ?