[Bug libstdc++/111138] views::zip_transform is underconstrained for empty range pack

[tkaminsk at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111138

--- Comment #2 from Tomasz Kamiński <tkaminsk at gcc dot gnu.org> ---
The issue is caused by the fact that we do not check `move_constructible<FD> &&
regular_invocable<FD&>` (required by [range.zip.transform] p2.1.1) for
`sizeof...(_Ts) == 0` case.
```
struct _ZipTransform
{
  template<typename _Fp, typename... _Ts>
    requires (sizeof...(_Ts) == 0) || __detail::__can_zip_transform_view<_Fp,
_Ts...>
    constexpr auto
    operator() [[nodiscard]] (_Fp&& __f, _Ts&&... __ts) const
    ...
};

```