13 Templates [temp]

13.4 Template arguments [temp.arg]

13.4.4 Template template arguments [temp.arg.template]

A template-argument for a template template-parameter shall be the name of a class template or an alias template, expressed as id-expression.
Only primary 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.spec.partial]) associated with the primary template are considered when a specialization based on the template template-parameter is instantiated.
If a specialization is not reachable from the point of instantiation, and it would have been selected had it been reachable, the program is ill-formed, no diagnostic required.
[Example 1: 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 P when P is at least as specialized as the template-argument A.
In this comparison, if P is unconstrained, the constraints on A are not considered.
If P contains a template parameter pack, then A also matches P if each of A's template parameters matches the corresponding template parameter in the template-head 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-head 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-head 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 2: template<class T> class A { /* ... */ }; template<class T, class U = T> class B { /* ... */ }; template<class ... Types> class C { /* ... */ }; template<auto n> class D { /* ... */ }; template<template<class> class P> class X { /* ... */ }; template<template<class ...> class Q> class Y { /* ... */ }; template<template<int> class R> class Z { /* ... */ }; X<A> xa; // OK X<B> xb; // OK X<C> xc; // OK Y<A> ya; // OK Y<B> yb; // OK Y<C> yc; // OK Z<D> zd; // OK — end example]
[Example 3: 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]
[Example 4: template<typename T> concept C = requires (T t) { t.f(); }; template<typename T> concept D = C<T> && requires (T t) { t.g(); }; template<template<C> class P> struct S { }; template<C> struct X { }; template<D> struct Y { }; template<typename T> struct Z { }; S<X> s1; // OK, X and P have equivalent constraints S<Y> s2; // error: P is not at least as specialized as Y S<Z> s3; // OK, P is at least as specialized as Z — end example]
A template template-parameter P is at least as specialized as a template template-argument A if, given the following rewrite to two function templates, the function template corresponding to P is at least as specialized as the function template corresponding to A according to the partial ordering rules for function templates.
Given an invented class template X with the template-head of A (including default arguments and requires-clause, if any):
  • Each of the two function templates has the same template parameters and requires-clause (if any), respectively, as P or A.
  • Each function template has a single function parameter whose type is a specialization of X with template arguments corresponding to the template parameters from the respective function template where, for each template parameter PP in the template-head of the function template, a corresponding template argument AA is formed.
    If PP declares a template parameter pack, then AA is the pack expansion PP... ([temp.variadic]); otherwise, AA is the id-expression PP.
If the rewrite produces an invalid type, then P is not at least as specialized as A.