That's because templates have no type-checking in C++. Ideally, you wouldn't be able to do this with concepts, but I don't know whether that's true for the con

That's because templates have no type-checking in C++. Ideally, you wouldn't be able to do this with concepts, but I don't know whether that's true for the concepts that finally made it into the standard (i.e. I don't know if it would be an error to call a function or operator on an argument if that function isn't required in the concept specification.).