CWG2680 [over.class.match.deduct] CTAD for aggregates with designated initializers

[cmeerw]

Full name of submitter (unless configured in github; will be published with the issue): Christof Meerwald

Reference (section label): over.class.match.deduct

Link to reflector thread (if any): CTAD for aggregates with designated initializers

Issue description: Examples like

  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

result in unexpected deduced types as CTAD doesn't limit deduction to the aggregate deduction candidate when a designated initializer list is used.

Suggested resolution: see Re: CTAD for aggregates with designated initializers

over.match.list should consider only the aggregate deduction candidate (if any) when given a designated-initializer-list.

also Re: CTAD for aggregates with designated initializers

[jensmaurer]

CWG2680