From 4513f0f0224fd34bbebfdc7aa6bb78a995a6ac7b Mon Sep 17 00:00:00 2001
From: Zhihao Yuan <zy@miator.net>
Date: Thu, 20 Jun 2019 22:09:40 +0000
Subject: [libc++] Recommit r363692 to implement P0608R3

Re-apply the change which was reverted in r363764 as-is after
breakages being resolved.  Thanks Eric Fiselier for working
hard on this.

See also: https://bugs.llvm.org/show_bug.cgi?id=42330

Differential Revision: https://reviews.llvm.org/D44865

llvm-svn: 363993
---
 libcxx/include/variant | 32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

(limited to 'libcxx/include/variant')

diff --git a/libcxx/include/variant b/libcxx/include/variant
index 5d0722b6296..baf163050dd 100644
--- a/libcxx/include/variant
+++ b/libcxx/include/variant
@@ -1098,11 +1098,39 @@ struct __overload<> { void operator()() const; };
 template <class _Tp, class... _Types>
 struct __overload<_Tp, _Types...> : __overload<_Types...> {
   using __overload<_Types...>::operator();
-  __identity<_Tp> operator()(_Tp) const;
+
+  static auto __test(_Tp (&&)[1]) -> __identity<_Tp>;
+
+  template <class _Up>
+  auto operator()(_Tp, _Up&& __t) const
+      -> decltype(__test({ _VSTD::forward<_Up>(__t) }));
+};
+
+template <class _Base, class _Tp>
+struct __overload_bool : _Base {
+  using _Base::operator();
+
+  template <class _Up, class _Ap = __uncvref_t<_Up>>
+  auto operator()(bool, _Up&&) const
+      -> enable_if_t<is_same_v<_Ap, bool>, __identity<_Tp>>;
 };
 
+template <class... _Types>
+struct __overload<bool, _Types...>
+    : __overload_bool<__overload<_Types...>, bool> {};
+template <class... _Types>
+struct __overload<bool const, _Types...>
+    : __overload_bool<__overload<_Types...>, bool const> {};
+template <class... _Types>
+struct __overload<bool volatile, _Types...>
+    : __overload_bool<__overload<_Types...>, bool volatile> {};
+template <class... _Types>
+struct __overload<bool const volatile, _Types...>
+    : __overload_bool<__overload<_Types...>, bool const volatile> {};
+
 template <class _Tp, class... _Types>
-using __best_match_t = typename result_of_t<__overload<_Types...>(_Tp&&)>::type;
+using __best_match_t =
+    typename invoke_result_t<__overload<_Types...>, _Tp, _Tp>::type;
 
 } // __variant_detail
 
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