[Bug c++/102529] New: ctad for aliases fails in the presence of constraints

["barry.revzin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102529

            Bug ID: 102529
           Summary: ctad for aliases fails in the presence of constraints
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Here's an example:

template <typename T>
struct C {
    template <typename U>
    C(U);
};

template <typename U>
C(U) -> C<U*>;

template <typename T>
    requires true
using A = C<T>;

C ok(1);   // ok, a is a C<int*>
A bad(2);  // fails

The provided error on gcc 11.2 (and trunk as of this writing) is:

<source>:15:8: error: class template argument deduction failed:
   15 | A bad(2);  // fails
      |        ^
<source>:15:8: error: no matching function for call to 'C(int)'
<source>:2:8: note: candidate: 'template<class T> C(C<T>)-> C<T> requires 
__is_same(A<T>, C<T>)'
    2 | struct C {
      |        ^
<source>:2:8: note:   template argument deduction/substitution failed:
<source>:15:8: note:   mismatched types 'C<T>' and 'int'
   15 | A bad(2);  // fails
      |        ^
<source>:4:5: note: candidate: 'template<class T, class U> C(U)-> C<T> requires
 __is_same(A<T>, C<T>)'
    4 |     C(U);
      |     ^
<source>:4:5: note:   template argument deduction/substitution failed:
<source>:15:8: note:   couldn't deduce template parameter 'T'
   15 | A bad(2);  // fails
      |        ^

If you drop the "requires true" part of the alias template A, then this works.
As far as I understand the rules, this should work. MSVC accepts the example.
clang does not yet implement class template argument deduction for alias
templates.