[Bug c++/103747] New: Passing parameter pack expansion to alias template with one argument fails as function return type

[Bug c++/103747] New: Passing parameter pack expansion to alias template with one argument fails as function return type

[info@ebner-markus.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103747

            Bug ID: 103747
           Summary: Passing parameter pack expansion to alias template
                    with one argument fails as function return type
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: info@ebner-markus.de
  Target Milestone: ---

GCC: 11.2 / trunk
CLANG: 13

The following code does not compile with GCC, but compiles fine with clang:
https://godbolt.org/z/vP5YMKqKe

        #include <array>

        template<template <typename...> typename TClass, typename... TArgs>
        TClass<TArgs...> factory() {
                return TClass<TArgs...>();
        }

        template<typename TItem>
        using FixedSize5Array = std::array<TItem, 5>;

        int main() {
                auto arr = factory<FixedSize5Array, int>();
                return 0;
        }

Changing the alias template to the following, makes the code compile with GCC,
but not with clang: https://godbolt.org/z/r45Ydx65q

        template<typename... TItem>
        using FixedSize5Array = std::array<TItem..., 5>;

To get it to compile with both compilers, take the first version, and replace
the explicit return type of factory() with auto:
https://godbolt.org/z/WjYrWbq18

[Bug c++/103747] Passing parameter pack expansion to alias template with one argument fails as function return type

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103747

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
CWG DR 1430 does look similar to this issue really:
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1430

template type alias is a mess.