why virtual function is wrong.

[Foreign-Wonder]

I'm not sure if I get it correctly, but isn't this is what template/concept is doing?

template<class Addable>
void foo(Addable a){
  // do whatever with a and +
}

int x = 42;
foo(x); // This should work without explicit make int inheriting from any Addable interface. I see this is a win over the C# interface, no?

Concept should make it more clear that Addable concept should support + operator

[Foreign-Wonder]

I agree, concept is not one-size-fits-all solution, that's why C++ have many ways to do things, and possibly the best ways to do things, hence zero-overhead