[Bug c++/105841] [12/13 Regression] Change in behavior of CTAD for alias templates

["ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105841

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=103852
      Known to work|                            |11.3.0
      Known to fail|                            |12.1.0, 13.0
   Last reconfirmed|                            |2022-06-06
                 CC|                            |jason at gcc dot gnu.org,
                   |                            |ppalka at gcc dot gnu.org
   Target Milestone|---                         |12.2
            Summary|Change in behavior of CTAD  |[12/13 Regression] Change
                   |for alias templates         |in behavior of CTAD for
                   |                            |alias templates
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |rejects-valid
     Ever confirmed|0                           |1

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
We started rejecting the testcase after r12-7984-ga11f204e5a30d1.

Reduced rejects-valid example:

template<class T, int N>
struct A { A(...); };

template<class T, class... Ts>
A(T, Ts...) -> A<T, sizeof...(Ts)>;

template<class T, int N=0>
using B = A<T, N>;

B b(0, 0);

Removing B's default template argument makes us accept again.