Added a workaround for a `-fdelayed-template-parsing` bug.

[mpark]

Summary

There seems to be an additional bug in -fdelayed-template-parsing
similar to
http://llvm.org/viewvc/llvm-project?view=revision&revision=236063.

This is a workaround for it for <variant> to compile with clang-cl on Windows.

[mpark]

Fixed a variable name.

[EricWF]

LGTM!

Make sure to remove the // UNSUPPORTED: windows in the visit.pass.cpp test before committing.

[EricWF]

include/variant

483

Oh this should also use _VSTD::addressof

[mpark]

include/variant

483

I did this just because of https://godbolt.org/g/4yYeXu, but I'll add it for now.

[mpark]

• Removed the // UNSUPPORTED: windows in visit.pass.cpp.
• Used _VSTD::addressof.

include/variant

private:
   template <class... _Fs>
   inline _LIBCPP_INLINE_VISIBILITY
   static constexpr auto __make_farray(_Fs&&... __fs) {
     __std_visit_visitor_return_type_check<decay_t<_Fs>...>();
     using __result = array<common_type_t<decay_t<_Fs>...>, sizeof...(_Fs)>;
     return __result{{_VSTD::forward<_Fs>(__fs)...}};
   }
 
-  template <class _Fp, class... _Vs, size_t... _Is>
-  inline _LIBCPP_INLINE_VISIBILITY
-  static constexpr auto __make_dispatch(index_sequence<_Is...>) {
-    struct __dispatcher {
-      static constexpr decltype(auto) __dispatch(_Fp __f, _Vs... __vs) {
+  template <std::size_t... _Is>
+    template <class _Fp, class... _Vs>
+    inline _LIBCPP_INLINE_VISIBILITY
         return __invoke_constexpr(
             static_cast<_Fp>(__f),
             __access::__base::__get_alt<_Is>(static_cast<_Vs>(__vs))...);
-      }
-    };
-    return _VSTD::addressof(__dispatcher::__dispatch);
+
+  template <class _Fp, class... _Vs, size_t... _Is>
+  inline _LIBCPP_INLINE_VISIBILITY
+  static constexpr auto __make_dispatch(index_sequence<_Is...>) {
+    return &__dispatcher<Is...>::template __dispatch<_Fp, _Vs...>;
   }
 
   template <size_t _Ip, class _Fp, class... _Vs>
   inline _LIBCPP_INLINE_VISIBILITY
   static constexpr auto __make_fdiagonal_impl() {
     return __make_dispatch<_Fp, _Vs...>(
         index_sequence<(__identity<_Vs>{}, _Ip)...>{});
   }