[libcxx] Make sure reference_wrapper works with incomplete types

Summary: Completes P0357R3, which was merged into the C++20 Working Draft in San Diego.

Reviewers: EricWF, mclow.lists

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D54722

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@357423 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/refwrap/refwrap.const/ctor.incomplete.pass.cpp

@@ -0,0 +1,41 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+
+// <functional>
+//
+// reference_wrapper<T>
+//
+//  where T is an incomplete type (since C++20)
+
+
+#include <functional>
+#include <cassert>
+
+
+struct Foo;
+
+Foo& get_foo();
+
+void test() {
+    Foo& foo = get_foo();
+    std::reference_wrapper<Foo> ref{foo};
+    assert(&ref.get() == &foo);
+}
+
+struct Foo { };
+
+Foo& get_foo() {
+    static Foo foo;
+    return foo;
+}
+
+int main() {
+    test();
+}