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 or primary variable 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 ]
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. Two template parameters match if they are of the same kind (type, non-type, template), for non-type template-parameters, their types are equivalent ([temp.over.link]), and for template template-parameters, each of their corresponding template-parameters matches, recursively. 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> 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 ]
[ 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 ]