Partial template specialization ambiguity

Question

I cant see why the statement in main is ambiguous.

template struct X
{ void f() { cout << \"Primary template\" << endl         


        

Answer 1

A template specialization is more specialized than another if every argument list that matches the first also matches the second, but not the other way around.

When looking at X and X:

• X matches the first but not the second.
• X matches the second but not the first.

Therefore neither is more specialized than the other, and a template instantiation that matches both specializations won't compile.