14 Templates [temp]

A template-argument for a template template-parameter shall be the name of a class template or an alias template, expressed as id-expression. When the template-argument names a class template, only primary class templates are considered when matching the template template argument with the corresponding parameter; partial specializations are not considered even if their parameter lists match that of the template template parameter.

Any partial specializations ([temp.class.spec]) associated with the primary class template are considered when a specialization based on the template template-parameter is instantiated. If a specialization is not visible at the point of instantiation, and it would have been selected had it been visible, the program is ill-formed; no diagnostic is required. [ Example:

template<class T> class A {     // primary template
  int x;
};
template<class T> class A<T*> { // partial specialization
  long x;
};
template<template<class U> class V> class C {
  V<int>  y;
  V<int*> z;
};
C<A> c;                         // V<int> within C<A> uses the primary template,
                                // so c.y.x has type int
                                // V<int*> within C<A> uses the partial specialization,
                                // so c.z.x has type long

 — end example ]

[ Example:

template<class T> class A { /* ... */ };
template<class T, class U = T> class B { /* ... */ };
template <class ... Types> class C { /* ... */ };

template<template<class> class P> class X { /* ... */ };
template<template<class ...> class Q> class Y { /* ... */ };

X<A> xa;            // OK
X<B> xb;            // ill-formed: default arguments for the parameters of a template argument are ignored
X<C> xc;            // ill-formed: a template parameter pack does not match a template parameter

Y<A> ya;            // OK
Y<B> yb;            // OK
Y<C> yc;            // OK

 — end example ]

A template-argument matches a template template-parameter (call it P) when each of the template parameters in the template-parameter-list of the template-argument's corresponding class template or alias template (call it A) matches the corresponding template parameter in the template-parameter-list of P. When P's template-parameter-list contains a template parameter pack ([temp.variadic]), the template parameter pack will match zero or more template parameters or template parameter packs in the template-parameter-list of A with the same type and form as the template parameter pack in P (ignoring whether those template parameters are template parameter packs) [ Example:

template <class T> struct eval;

template <template <class, class...> class TT, class T1, class... Rest>
struct eval<TT<T1, Rest...>> { };

template <class T1> struct A;
template <class T1, class T2> struct B;
template <int N> struct C;
template <class T1, int N> struct D;
template <class T1, class T2, int N = 17> struct E;

eval<A<int>> eA;                // OK: matches partial specialization of eval
eval<B<int, float>> eB;         // OK: matches partial specialization of eval
eval<C<17>> eC;                 // error: C does not match TT in partial specialization
eval<D<int, 17>> eD;            // error: D does not match TT in partial specialization
eval<E<int, float>> eE;         // error: E does not match TT in partial specialization

 — end example ]