Follow up build fix for P0655R1 visit<R>: Explicit Return Type for visit

[ldionne]

Will commit once CI passes.

[ldionne]

Ah, now I understand. This change is necessary to fix the build after https://reviews.llvm.org/D92044. The problem is that CI didn't run for that revision, so we didn't catch the failure. When you submit a patch, please make sure to follow the arc diff instructions here: https://llvm.org/docs/Phabricator.html#requesting-a-review-via-the-command-line.

Using arc diff makes sure that the right repository is populated in the Phabricator revision, and that allows CI to run. I know it's easy to miss (and would love for a more robust setup), but that's what we have for now. :-) I'll check this in now.

libcxx/include/variant

constexpr bool operator>=(const variant<_Types...>& __lhs,
   if (__rhs.valueless_by_exception()) return true;
   if (__lhs.valueless_by_exception()) return false;
   if (__lhs.index() > __rhs.index()) return true;
   if (__lhs.index() < __rhs.index()) return false;
   return __variant::__visit_value_at(
       __lhs.index(), __convert_to_bool<greater_equal<>>{}, __lhs, __rhs);
 }
 
-template <class _Visitor, class... _Vs>
+template <class... _Vs>
 inline _LIBCPP_INLINE_VISIBILITY
 _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
-constexpr void __throw_if_valueless(_Visitor&& __visitor, _Vs&&... __vs) {
+constexpr void __throw_if_valueless(_Vs&&... __vs) {
   const bool __valueless = (... || __vs.valueless_by_exception());
   if (__valueless) {
       __throw_bad_variant_access();
   }
 }
 
 template <class _Visitor, class... _Vs>
 inline _LIBCPP_INLINE_VISIBILITY
 _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
 constexpr decltype(auto) visit(_Visitor&& __visitor, _Vs&&... __vs) {
   using __variant_detail::__visitation::__variant;
-  _VSTD::__throw_if_valueless(_VSTD::forward<_Visitor>(__visitor),
-                              _VSTD::forward<_Vs>(__vs)...);
+  _VSTD::__throw_if_valueless(_VSTD::forward<_Vs>(__vs)...);
   return __variant::__visit_value(_VSTD::forward<_Visitor>(__visitor),
                                   _VSTD::forward<_Vs>(__vs)...);
 }
 
 #if _LIBCPP_STD_VER > 17
 template <class _Rp, class _Visitor, class... _Vs>
 inline _LIBCPP_INLINE_VISIBILITY
 _LIBCPP_AVAILABILITY_THROW_BAD_VARIANT_ACCESS
 constexpr _Rp visit(_Visitor&& __visitor, _Vs&&... __vs) {
   using __variant_detail::__visitation::__variant;
-  _VSTD::__throw_if_valueless(_VSTD::forward<_Visitor>(__visitor),
-                              _VSTD::forward<_Vs>(__vs)...);
+  _VSTD::__throw_if_valueless(_VSTD::forward<_Vs>(__vs)...);
   return __variant::__visit_value<_Rp>(_VSTD::forward<_Visitor>(__visitor),
                                        _VSTD::forward<_Vs>(__vs)...);
 }
 #endif
 
 struct _LIBCPP_TEMPLATE_VIS monostate {};
 
 inline _LIBCPP_INLINE_VISIBILITY