An example like the following would seem to be plausible:

  template<class T, T*> struct X { };
  // We want to partially specialize for all nullptrs...
  template<class T> struct X<T, nullptr> { ... }; // NOT OK

This is disallowed by the rule in bullet 8.2 of _N4868_.13.7.6 [temp.class.spec]:

• The type of a template parameter corresponding to a specialized non-type argument shall not be dependent on a parameter of the specialization.

(See also issues 1315, 1647, and 2033.)