[libcxx] Implement P0318: unwrap_ref_decay and unwrap_reference

Summary:
This was voted into C++20 in San Diego. Note that there was a revision
D0318R2 which did include unwrap_reference_t, but we mistakingly voted
P0318R1 into the C++20 Working Draft (which does not include
unwrap_reference_t). This patch implements D0318R2, which is what
we'll end up with in the Working Draft once this mistake has been
fixed.

Reviewers: EricWF, mclow.lists

Subscribers: christof, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D54485

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@348138 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/refwrap/unwrap_reference.pass.cpp

@@ -0,0 +1,51 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <functional>
+//
+// template <class T>
+// struct unwrap_reference;
+//
+// template <class T>
+// using unwrap_reference_t = typename unwrap_reference<T>::type;
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+
+#include <functional>
+#include <type_traits>
+
+
+template <typename T, typename Expected>
+void check_equal() {
+  static_assert(std::is_same_v<typename std::unwrap_reference<T>::type, Expected>);
+  static_assert(std::is_same_v<typename std::unwrap_reference<T>::type, std::unwrap_reference_t<T>>);
+}
+
+template <typename T>
+void check() {
+  check_equal<T, T>();
+  check_equal<T&, T&>();
+  check_equal<T const, T const>();
+  check_equal<T const&, T const&>();
+
+  check_equal<std::reference_wrapper<T>, T&>();
+  check_equal<std::reference_wrapper<T const>, T const&>();
+}
+
+struct T { };
+
+int main() {
+  check<T>();
+  check<int>();
+  check<float>();
+
+  check<T*>();
+  check<int*>();
+  check<float*>();
+}