Why is it disallowed for partial specialization in a non-type argument to use nested template parameters

Question

I have this code

template
struct A;

template
struct A {
  /* ... */
};

// should work
A<25&g         


        

Answer 1

Partial specialisation requires that the non-type template argument be resolvable at compile time.

At this point

template
struct A {
  /* ... */
};

N is a variable that can take more than one value and the compiler is unable to calculate N % 5 with certainty.

Your example instantiates a using

A<25> a;

but you could also have

A<25> a1;
A<15> a2;

How does the compiler choose a value for N in this scenario? It can't and so it has to disallow the code.