[Bug c++/105716] New: CTAD failure with member function returning auto as template argument

["j-l.wynen at hotmail dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105716

            Bug ID: 105716
           Summary: CTAD failure with member function returning auto as
                    template argument
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: j-l.wynen at hotmail dot de
  Target Milestone: ---

gcc 12 fails to compile the following code (tested 12.1 and trunk). Tried with
c++17 and c++20.

template <class T> struct Func {
  static auto apply() {}
};

template <class T>
struct A {
    T x;
};

template <class T>
A(T) -> A<T>;

int main() {
    A a{Func<double>::apply};
}

with an error of

error: 'A<auto (*)()> a' has incomplete type

The code compiles fine with gcc 11, 10, and 9 as well as clang and MSCV.

Changing static auto apply() {} to static void apply() {} or any other concrete
return type (and proper return statement) makes the code compiles.

Also, specifying the template argument of A in main makes it compile.

https://godbolt.org/z/c9cGnGEPq