How to enforce use of template specialization?

Question

I'm trying to make my template function produce a compile-time error if the non-specialized base version is instantiated. I tried the usual compile-time assert pattern (negative array size) but the compile is failing even when the template is not instantiated. Any thoughts on how to to make it fail if and only if the base-template function is instantiated?

template<class Foo> void func(Foo x) {
  // I want the compiler to complain only if this function is instantiated.
  // Instead, the compiler is complaining here at the declaration.
  int Must_Use_Specialization[-1];
}

template<> void func(int x) {
  printf("Hi\n");
}

Answer 1

Not defining it is the easiest solution:

template<class Foo> void func(Foo x);

template<> void func(int x) {
  printf("Hi\n");
}

You can also define them in CPP files and use this which will work.

And static assert method:

template<class Foo> void func(Foo x) {
  static_assert(sizeof(Foo) != sizeof(Foo), "func must be specialized for this type!");
}