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

2680. Class template argument deduction for aggregates with designated initializers

Consider:

   template<typename T>
   struct X
   {
    int i{};
    T t;
   };

   X x{ .t = X<int>{ 1, 2 } }; // should deduce X<X<int>> (not X<int>)

and

   template<typename T>
   struct Y
   {
    int i;
    T t;
    int j;
   };

   template<typename T>
   Y(int, T) -> Y<T>;

   Y y{ .t = 'c', .j = 2 }; // should be ill-formed

Class template argument deduction does not limit deduction to the aggregate deduction candidate when a designated initializer list is used.

Suggested resolution:

1. Change in 12.2.2.9 [over.match.class.deduct] paragraph 1 as follows:

   ... If the initializer has a designated-initializer-list, any guide that is not an aggregate deduction candidate is not viable; for an aggregate deduction candidate, the designated-initializer-list is replaced by an initializer-list with the same elements. In addition, if C is defined and inherits constructors (9.9 [namespace.udecl]) from a direct base class ...

2. Change in 12.2.2.9 [over.match.class.deduct] bullet 3.5 as follows:

   • ...
   • If f was generated from a deduction-guide (13.7.2.3 [temp.deduct.guide]), then f' is considered to be so as well.
   • If f was generated from an aggregate deduction guide, then f' is considered to be so as well.