This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-10-26

2848. Omitting an empty template argument list for explicit instantiation

[Accepted as a DR at the March, 2024 meeting.]

(From submission #489.)

Subclause 13.9.3 [temp.explicit] paragraph 8 specifies:

A trailing template-argument can be left unspecified in an explicit instantiation of a function template specialization or of a member function template specialization provided it can be deduced (13.10.3.7 [temp.deduct.decl]). If all template arguments can be deduced, the empty template argument list <> may be omitted. [Example 3:

  template<class T> class Array { /* ... */ };
  template<class T> void sort(Array<T>& v) { /* ... */ }

  // instantiate sort(Array<int>&) -- template-argument deduced
  template void sort<>(Array<int>&);

—end example]

This paragraph is redundant with a more general provision on explicitly specifying template arguments in 13.10.2 [temp.arg.explicit] paragraph 4:

Trailing template arguments that can be deduced (13.10.3 [temp.deduct]) or obtained from default template-arguments may be omitted from the list of explicit template-arguments. [Note 1: A trailing template parameter pack (13.7.4 [temp.variadic]) not otherwise deduced will be deduced as an empty sequence of template arguments. —end note] If all of the template arguments can be deduced or obtained from default template-arguments, they may all be omitted; in this case, the empty template argument list <> itself may also be omitted. ...

A similar duplication is in 13.9.4 [temp.expl.spec] paragraph 10.

Proposed resolution (approved by CWG 2024-02-16):

1. Change the example in 13.9.3 [temp.explicit] paragraph 4 as follows:

     ...
     template<class T> void sort(Array<T>& v) { /* ... */ }
     template void sort(Array<char>&);  // argument is deduced here (13.10.2 [temp.arg.explicit])
     ...
   

2. Remove 13.9.3 [temp.explicit] paragraph 8:

   A trailing template-argument can be left unspecified in an explicit instantiation of a function template specialization or of a member function template specialization provided it can be deduced (13.10.3.7 [temp.deduct.decl]). If all template arguments can be deduced, the empty template argument list <> may be omitted. [Example 3:

     template<class T> class Array { /* ... */ };
     template<class T> void sort(Array<T>& v) { /* ... */ }
   
     // instantiate sort(Array<int>&) -- template-argument deduced
     template void sort<>(Array<int>&);
   

   —end example]

3. Change the example in 13.9.4 [temp.expl.spec] paragraph 1 as follows:

   ... [ Example:

     template<class T> class stream;
     template<> class stream<char> { /* ... */ }; // #1
   
     template<class T> class Array { /* ... */ };
     template<class T> void sort(Array<T>& v) { /* ... */ }
   
     template<> void sort<int>(Array<int>&);   // #2
     template<> void sort<char*>(Array<char*>&);   // #3 template argument is deduced (13.10.2 [temp.arg.explicit])
     ...
   

   Given these declarations, stream<char> #1 will be used as the definition of streams of chars; other streams will be handled by class template specializations instantiated from the class template. Similarly, #2 will be used as the sort function for arguments of type Array<int> and #3 sort<char*> will be used as the sort function for arguments of type Array<char*>; other Array types will be sorted by functions generated from the function template. —end example]

4. Remove 13.9.4 [temp.expl.spec] paragraph 10:

   A trailing template-argument can be left unspecified in the template-id naming an explicit function template specialization provided it can be deduced (13.10.3.7 [temp.deduct.decl]). [Example 6:

     template<class T> class Array { /* ... */ };
     template<class T> void sort(Array<T>& v);
   
     // explicit specialization for sort(Array<int>&)
     // with deduced template-argument of type int
     template<> void sort(Array<int>&);
   

   —end example]