[Bug c++/116005] New: Class template argument deduction for alias templates error in C++20

[raplonu.jb at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116005

            Bug ID: 116005
           Summary: Class template argument deduction for alias templates
                    error in C++20
           Product: gcc
           Version: 14.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: raplonu.jb at gmail dot com
  Target Milestone: ---

Hi, the following code does not compile with gcc 14 & c++20.

template <typename T1, typename T2>
struct A {
    template <typename U>
    A(const std::vector<U> &arr) {}
};

template <typename T1, typename T2>
A(std::vector<T1> const &) -> A<const T1, T2>;

template <typename T>
using B = A<T, void>;

int main() {
    const std::vector<int> v;

    B b{v};
}

It gives the following error message:

<source>:19:10: error: class template argument deduction failed:
   19 |     B b{v};
      |          ^
<source>:19:10: error: no matching function for call to 'A(const
std::vector<int>&)'
<source>:11:1: note: candidate: 'template<class T1, class T2> A(const
std::vector<T1>&)-> A<const T1, T2> requires  __is_deducible (B, A<const T1,
T2>)'
   11 | A(std::vector<T1> const &) -> A<const T1, T2>;
      | ^
<source>:11:1: note:   template argument deduction/substitution failed:
<source>:19:10: note:   couldn't deduce template parameter 'T2'
   19 |     B b{v};
      |          ^

I believe this code is valid and should compile.

Also, I notice with gcc 13 and above, it works if I replace `const T1` by
`std::add_const_t<T1>` in the deduction guide (or if I remove all const
keywords)

repro: https://godbolt.org/z/E5fE53oe1

Thanks for your time