CWG2664 [over.match.class.deduct] CTAD for alias template wording doesn't match design intent

[cmeerw]

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

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

Link to reflector thread (if any): Somewhat surprising CTAD for alias template behaviour

Issue description: [over.match.class.deduct]/3 only has an exception for deduction failure for non-deduced contexts when deducing the return type from the defining-type-id, but not for other cases where deduction fails according to [temp.deduct.type]/2, e.g.

  template <class S1, class S2> struct C {
    C(...);
  };

  template<class T1> C(T1) -> C<T1, T1>;
  template<class T1, class T2> C(T1, T2) -> C<T1 *, T2>;

  template<class V1, class V2> using A = C<V1, V2>;

resulting in A having neither of these deduction guides.

Suggested resolution: see Re: Somewhat surprising CTAD for alias template behaviour

[jensmaurer]

CWG2664