The explicitly specialized primary template class shall supress consideration of other partial specialization

[xmh0511]

Bugzilla Link	51709
Version	trunk
OS	All
CC	@DougGregor,@zygoloid

Extended Description

template<class T> struct A {
  template<class T2> struct B {
     static const int value = 1;
  };                               // #&#8203;1
  template<class T2> struct B<T2*> {
     static const int value = 2;
  };                          // #&#8203;2
};

template<> template<class T2> struct A<short>::B {
    static const int value = 3;
};            // #&#8203;3
int main(){
   std::cout<< A<short>::B<int*>::value;  // shall be 3
}

Clang give 2, as per [temp.spec.partial.member] p2.

If a member template of a class template is partially specialized, the member template partial specializations are member templates of the enclosing class template; if the enclosing class template is instantiated ([temp.inst], [temp.explicit]), a declaration for every member template partial specialization is also instantiated as part of creating the members of the class template specialization. If the primary member template is explicitly specialized for a given (implicit) specialization of the enclosing class template, the partial specializations of the member template are ignored for this specialization of the enclosing class template. If a partial specialization of the member template is explicitly specialized for a given (implicit) specialization of the enclosing class template, the primary member template and its other partial specializations are still considered for this specialization of the enclosing class template.

We explicitly specialized the primary template class B for enclosing class template specialization A<short>, hence the other partial specialization shall not be considered anymore.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (8084cac4-606c-41d3-baea-106d693787ba)

| | | | --- | --- | | Bugzilla Link | [51709](https://llvm.org/bz51709) | | Version | trunk | | OS | All | | CC | @DougGregor,@zygoloid |

Extended Description

template<class T> struct A {
  template<class T2> struct B {
     static const int value = 1;
  };                               // #&amp;#<!-- -->8203;1
  template&lt;class T2&gt; struct B&lt;T2*&gt; {
     static const int value = 2;
  };                          // #&amp;#<!-- -->8203;2
};

template&lt;&gt; template&lt;class T2&gt; struct A&lt;short&gt;::B {
    static const int value = 3;
};            // #&amp;#<!-- -->8203;3
int main(){
   std::cout&lt;&lt; A&lt;short&gt;::B&lt;int*&gt;::value;  // shall be 3
}

Clang give 2, as per [temp.spec.partial.member] p2.

> If a member template of a class template is partially specialized, the member template partial specializations are member templates of the enclosing class template; if the enclosing class template is instantiated ([temp.inst], [temp.explicit]), a declaration for every member template partial specialization is also instantiated as part of creating the members of the class template specialization. If the primary member template is explicitly specialized for a given (implicit) specialization of the enclosing class template, the partial specializations of the member template are ignored for this specialization of the enclosing class template. If a partial specialization of the member template is explicitly specialized for a given (implicit) specialization of the enclosing class template, the primary member template and its other partial specializations are still considered for this specialization of the enclosing class template.

We explicitly specialized the primary template class B for enclosing class template specialization A<short>, hence the other partial specialization shall not be considered anymore.

[cor3ntin]

https://compiler-explorer.com/z/Ea3Gs3jzP

@sdkrystian @mizvekov

This is... not great!