Question

By looking at the following code, I am confused by line 3.
Line 3 is not a special case of the base template, it is more like a “class overload”. But it can be compiled successfully.
The obj1 in line 7 is defined according to line 3, but failed to compile.
How come?

template<typename S,int T, void(* U)()> class Bar{};  // Base template
template<int T, void(* U)()> class Bar<double, T, U>{}; // Specialization, which is good
template<int T, void(* U)()> class Bar<double, U, T>{}; // Also good, how come?

void func(){};
int main(){
   //Bar<double, func, 1> obj1;   // Error, from line 3
}

Answer 1

As long as that form is not used anywhere – then the compiler will not complain. If it is used, and the compiler has to instantiate – then it will complain. Which is why if you uncomment, you get the error – at this point, the compiler sees the flawed partial specialization.