From Jason Turner

[temp.arg.template]

C++20 → C++23 2.5 KB 25 lines
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  ### Template template arguments <a id="temp.arg.template">[[temp.arg.template]]</a>
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  A *template-argument* for a template *template-parameter* shall be the
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  name of a class template or an alias template, expressed as
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- *id-expression*. When the *template-argument* names a class template,
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- only primary class templates are considered when matching the template
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- template argument with the corresponding parameter; partial
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  specializations are not considered even if their parameter lists match
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  that of the template template parameter.
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- Any partial specializations [[temp.class.spec]] associated with the
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- primary class template or primary variable template are considered when
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- a specialization based on the template *template-parameter* is
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- instantiated. If a specialization is not visible at the point of
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- instantiation, and it would have been selected had it been visible, the
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- program is ill-formed, no diagnostic required.
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  [*Example 1*:
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  ``` cpp
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  template<class T> class A { // primary template
@@ -85,12 +84,12 @@ template <class T1> struct A;
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  template <class T1, class T2> struct B;
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  template <int N> struct C;
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  template <class T1, int N> struct D;
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  template <class T1, class T2, int N = 17> struct E;
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- eval<A<int>> eA; // OK: matches partial specialization of eval
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- eval<B<int, float>> eB; // OK: matches partial specialization of eval
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  eval<C<17>> eC; // error: C does not match TT in partial specialization
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  eval<D<int, 17>> eD; // error: D does not match TT in partial specialization
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  eval<E<int, float>> eE; // error: E does not match TT in partial specialization
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  ```
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  ### Template template arguments <a id="temp.arg.template">[[temp.arg.template]]</a>
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  A *template-argument* for a template *template-parameter* shall be the
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  name of a class template or an alias template, expressed as
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+ *id-expression*. Only primary templates are considered when matching the
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+ template template argument with the corresponding parameter; partial
 
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  specializations are not considered even if their parameter lists match
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  that of the template template parameter.
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+ Any partial specializations [[temp.spec.partial]] associated with the
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+ primary template are considered when a specialization based on the
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+ template *template-parameter* is instantiated. If a specialization is
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+ not reachable from the point of instantiation, and it would have been
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+ selected had it been reachable, the program is ill-formed, no diagnostic
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+ required.
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  [*Example 1*:
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  ``` cpp
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  template<class T> class A { // primary template
 
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  template <class T1, class T2> struct B;
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  template <int N> struct C;
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  template <class T1, int N> struct D;
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  template <class T1, class T2, int N = 17> struct E;
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+ eval<A<int>> eA; // OK, matches partial specialization of eval
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+ eval<B<int, float>> eB; // OK, matches partial specialization of eval
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  eval<C<17>> eC; // error: C does not match TT in partial specialization
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  eval<D<int, 17>> eD; // error: D does not match TT in partial specialization
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  eval<E<int, float>> eE; // error: E does not match TT in partial specialization
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  ```
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