This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 110b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2023-01-15


1847. Clarifying compatibility during partial ordering

Section: 13.10.3.6  [temp.deduct.type]     Status: CD4     Submitter: Jason Merrill     Date: 2014-01-30

[Moved to DR at the October, 2015 meeting.]

There is implementation divergence in the handling of an example like

  template<typename D> struct A { };
  template<typename T> struct Wrap1 { typedef T type; };
  template<typename T> struct Wrap2 { typedef T type; };

  template<typename T1>
  A<typename Wrap1<T1>::type>
  fn(const A<T1>& x, const A<T1>& y);

  template<typename T2, typename U>
  A<typename Wrap2<T2>::type>
  fn(const A<T2>& x, const A<U>& y);

  A<int> (*p)(const A<int>&, const A<int>&) = fn;

The implementations that accept this example do so by not comparing the return types of the two templates during partial ordering, which seems to make sense given that partial ordering would not have been performed if the candidate specializations were not indistinguishable from the perspective of overload resolution. However, the existing wording is not clear that that is how such types are be handled.

Proposed resolution (May, 2015):

This issue is resolved by the resolution of issue 1391.