Title
Partial ordering of variadic class template partial specializations
Status
c++11
Section
13.10.3.6 [temp.deduct.type]
Submitter
Doug Gregor

Created on 2008-04-16.00:00:00 last changed 96 months ago

Messages

Date: 2011-03-15.00:00:00

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

Date: 2009-03-15.00:00:00

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 Pi of the respective template argument list of P is compared with the corresponding argument Ai 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 Pi is a pack expansion, then the pattern of Pi 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 Pi. During partial ordering (13.10.3.5 [temp.deduct.partial]), if Ai was originally a pack expansion and Pi is not a pack expansion, or if P does not contain a template argument corresponding to Ai, argument deduction fails.

Similarly, if P has a form that contains (T), then each parameter type Pi of the respective parameter-type-list of P is compared with the corresponding parameter type Ai of the corresponding parameter-type-list of A. If the parameter-declaration corresponding to Pi 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 Ai was originally a function parameter pack and Pi is not a function parameter pack, or if P does not contain a function parameter type corresponding to Ai, 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]

Date: 2009-08-03.00:00:00

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.)

History
Date User Action Args
2014-03-03 00:00:00adminsetstatus: fdis -> c++11
2011-04-10 00:00:00adminsetmessages: + msg3402
2011-04-10 00:00:00adminsetstatus: drafting -> fdis
2009-11-08 00:00:00adminsetstatus: review -> drafting
2009-03-23 00:00:00adminsetmessages: + msg1934
2008-06-29 00:00:00adminsetstatus: open -> review
2008-04-16 00:00:00admincreate