[libcxx] Make sure reference_wrapper works with incomplete types

[ldionne]

Summary

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

[ldionne]

Rebase onto master and update the licenses to the new license. Ping!

[ldionne]

I committed without a review since this is only adding tests.

libcxx/test/std/utilities/function.objects/refwrap/refwrap.helpers/cref.incomplete.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+
+// <functional>
+//
+// reference_wrapper
+//
+// template <ObjectType T> reference_wrapper<const T> cref(const T& t);
+//
+//  where T is an incomplete type (since C++20)
+
+#include <functional>
+#include <cassert>
+
+
+struct Foo;
+
+Foo& get_foo();
+
+void test() {
+    Foo const& foo = get_foo();
+    std::reference_wrapper<Foo const> ref = std::cref(foo);
+    assert(&ref.get() == &foo);
+}
+
+struct Foo { };
+
+Foo& get_foo() {
+    static Foo foo;
+    return foo;
+}
+
+int main() {
+    test();
+}

libcxx/test/std/utilities/function.objects/refwrap/refwrap.helpers/ref.incomplete.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+
+// <functional>
+//
+// reference_wrapper
+//
+// template <ObjectType T> reference_wrapper<T> ref(T& 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 = std::ref(foo);
+    assert(&ref.get() == &foo);
+}
+
+struct Foo { };
+
+Foo& get_foo() {
+    static Foo foo;
+    return foo;
+}
+
+int main() {
+    test();
+}

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

+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// 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();
+}

libcxx/test/std/utilities/function.objects/refwrap/refwrap.invoke/invoke.incomplete.fail.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+
+// <functional>
+//
+// reference_wrapper
+//
+// template <class... ArgTypes>
+//  std::invoke_result_t<T&, ArgTypes...>
+//      operator()(ArgTypes&&... args) const;
+//
+// Requires T to be a complete type (since C++20).
+
+#include <functional>
+
+
+struct Foo;
+Foo& get_foo();
+
+void test() {
+    std::reference_wrapper<Foo> ref = get_foo();
+    ref(0); // incomplete at the point of call
+}
+
+struct Foo { void operator()(int) const { } };
+Foo& get_foo() { static Foo foo; return foo; }
+
+int main() {
+    test();
+}

libcxx/www/cxx2a_status.html

-->
   <tr><td><a href="https://wg21.link/P0941R2">P0941R2</a></td><td>CWG</td><td>Integrating feature-test macros into the C++ WD</td><td>Rapperswil</td><td><i>In progress</i></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P1023R0">P1023R0</a></td><td>LWG</td><td>constexpr comparison operators for std::array</td><td>Rapperswil</td><td>Complete</td><td>8.0</td></tr>
   <tr><td><a href="https://wg21.link/P1025R1">P1025R1</a></td><td>CWG</td><td>Update The Reference To The Unicode Standard</td><td>Rapperswil</td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P1120R0">P1120R0</a></td><td>CWG</td><td>Consistency improvements for &lt;=&gt; and other comparison operators</td><td>Rapperswil</td><td></td><td></td></tr>
 
    <tr><td></td><td></td><td></td><td></td><td></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0318R1">P0318R1</a></td><td>LWG</td><td>unwrap_ref_decay and unwrap_reference</td><td>San Diego</td><td><i> </i></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0356R5">P0356R5</a></td><td>LWG</td><td>Simplified partial function application</td><td>San Diego</td><td><i> </i></td><td></td></tr>
-  <tr><td><a href="https://wg21.link/P0357R3">P0357R3</a></td><td>LWG</td><td>reference_wrapper for incomplete types</td><td>San Diego</td><td><i> </i></td><td></td></tr>
+  <tr><td><a href="https://wg21.link/P0357R3">P0357R3</a></td><td>LWG</td><td>reference_wrapper for incomplete types</td><td>San Diego</td><td>Complete</td><td>8.0</td></tr>
   <tr><td><a href="https://wg21.link/P0482R6">P0482R6</a></td><td>CWG</td><td>char8_t: A type for UTF-8 characters and strings</td><td>San Diego</td><td><i> </i></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0487R1">P0487R1</a></td><td>LWG</td><td>Fixing operator&gt;&gt;(basic_istream&amp;, CharT*) (LWG 2499)</td><td>San Diego</td><td><i> </i></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0591R4">P0591R4</a></td><td>LWG</td><td>Utility functions to implement uses-allocator construction</td><td>San Diego</td><td><i> </i></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0595R2">P0595R2</a></td><td>CWG</td><td>P0595R2 std::is_constant_evaluated()</td><td>San Diego</td><td><i> </i></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0602R4">P0602R4</a></td><td>LWG</td><td>variant and optional should propagate copy/move triviality</td><td>San Diego</td><td><i> </i></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0608R3">P0608R3</a></td><td>LWG</td><td>A sane variant converting constructor</td><td>San Diego</td><td><i> </i></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0655R1">P0655R1</a></td><td>LWG</td><td>visit&lt;R&gt;: Explicit Return Type for visit</td><td>San Diego</td><td><i> </i></td><td></td></tr>
   <tr><td><a href="https://wg21.link/P0771R1">P0771R1</a></td><td>LWG</td><td>std::function move constructor should be noexcept</td><td>San Diego</td><td><i> </i></td><td></td></tr>