[Bug c++/110856] New: GCC rejects template alias of function type as invalid template parameter

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110856

            Bug ID: 110856
           Summary: GCC rejects template alias of function type as invalid
                    template parameter
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

GCC rejects the following valid case:

#include <utility>
#include <type_traits>
#include <functional>

template<std::size_t, class T>
using identity = T;

template<class R, class Arg, std::size_t N>
using MakeFunType = decltype(
  []<std::size_t... Is>(std::index_sequence<Is...>) {
    return std::type_identity<R(identity<Is, Arg>...)>{};
  }(std::make_index_sequence<N>{})
)::type;

template<std::size_t N>
class S {
  using F1 = std::function<MakeFunType<void, int, 3>>; // ok
  using F2 = std::function<MakeFunType<void, int, N>>; // syntax error
};

https://godbolt.org/z/fc638b4Y1