This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

692. Partial ordering of variadic class template partial specializations

[Voted into the WP at the March, 2011 meeting as paper N3281.]

13.10.3.6 [temp.deduct.type] paragraph 22 describes how we cope with partial ordering between two function templates that differ because one has a function parameter pack while the other has a normal function parameter. However, this paragraph was meant to apply to template parameter packs as well, e.g., to help with partial ordering of class template partial specializations:

   template <class T1, class ...Z> class S; // #1
   template <class T1, class ...Z> class S<T1, const Z&...> {}; // #2
   template <class T1, class T2> class S<T1, const T2&> {};; // #3
   S<int, const int&> s; // both #2 and #3 match; #3 is more specialized

(See also issue 818.)

Proposed resolution (March, 2009):

Change 13.10.3.6 [temp.deduct.type] paragraphs 9-10 as follows (and add the example above to paragraph 9):

If P has a form that contains <T> or <i>, then each argument P_i of the respective template argument list of P is compared with the corresponding argument A_i of the corresponding template argument list of A. If the template argument list of P contains a pack expansion that is not the last template argument, the entire template argument list is a non-deduced context. If P_i is a pack expansion, then the pattern of P_i is compared with each remaining argument in the template argument list of A. Each comparison deduces template arguments for subsequent positions in the template parameter packs expanded by P_i. During partial ordering (13.10.3.5 [temp.deduct.partial]), if A_i was originally a pack expansion and P_i is not a pack expansion, or if P does not contain a template argument corresponding to A_i, argument deduction fails.

Similarly, if P has a form that contains (T), then each parameter type P_i of the respective parameter-type-list of P is compared with the corresponding parameter type A_i of the corresponding parameter-type-list of A. If the parameter-declaration corresponding to P_i is a function parameter pack, then the type of its declarator-id is compared with each remaining parameter type in the parameter-type-list of A. Each comparison deduces template arguments for subsequent positions in the template parameter packs expanded by the function parameter pack. During partial ordering (13.10.3.5 [temp.deduct.partial]), if A_i was originally a function parameter pack and P_i is not a function parameter pack, or if P does not contain a function parameter type corresponding to A_i, argument deduction fails. [Note: A function parameter pack can only occur at the end of a parameter-declaration-list (9.3.4.6 [dcl.fct]). —end note]