[Bug c++/120495] New: error: non-template type 'coroutine_handle' used as a template

[lh_mouse at 126 dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120495

            Bug ID: 120495
           Summary: error: non-template type 'coroutine_handle' used as a
                    template
           Product: gcc
           Version: 15.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lh_mouse at 126 dot com
  Target Milestone: ---

This code fails to compile:

(https://godbolt.org/z/jc3cnvacz)
```
#include <coroutine>
#include <exception>

struct fire_and_forget {};

template <typename... Args>
struct std::coroutine_traits<fire_and_forget, Args...>
{
    struct promise_type
    {
        fire_and_forget get_return_object() const noexcept
        {
            return{};
        }

        void return_void() const noexcept
        {
        }

        suspend_never initial_suspend() const noexcept
        {
            return{};
        }

        suspend_never final_suspend() const noexcept
        {
            return{};
        }

        void unhandled_exception() const noexcept
        {
            std::terminate();
        }
    };
};

struct foo
{
    fire_and_forget bar()
    {
        co_await std::suspend_always{ };
    }

private:
    // comment out line below to fix error
    using coroutine_handle = std::coroutine_handle<>;
};

int main()
{
    foo{}.bar();
}
```


```
<source>: In member function 'fire_and_forget foo::bar()':
<source>:41:39: error: non-template type 'coroutine_handle' used as a template
   41 |         co_await std::suspend_always{ };
      |                                       ^
<source>:41:9: error: cannot instantiate a 'coroutine handle' for promise type
'void'
   41 |         co_await std::suspend_always{ };
      |         ^~~~~~~~
<source>:42:5: error: non-template type 'coroutine_handle' used as a template
   42 |     }
      |     ^
<source>:39:21: error: cannot instantiate a 'coroutine handle' for promise type
'void'
   39 |     fire_and_forget bar()
      |                     ^~~
```

Downstream reports:

[1] https://github.com/msys2/MINGW-packages/issues/24209
[2] https://github.com/microsoft/cppwinrt/issues/1490