[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

llvm-svn: 348138

6 files changed, 145 insertions, 36 deletions

diff --git a/libcxx/include/functional b/libcxx/include/functional
index 61c87b02c70..1bddb9fadf4 100644
--- a/libcxx/include/functional
+++ b/libcxx/include/functional
@@ -68,6 +68,11 @@ template <class T> reference_wrapper<const T> cref(const T& t) noexcept;
 template <class T> void cref(const T&& t) = delete;
 template <class T> reference_wrapper<const T> cref(reference_wrapper<T> t) noexcept;
 
+template <class T> struct unwrap_reference;                                       // since C++20
+template <class T> struct unwrap_ref_decay : unwrap_reference<decay_t<T>> { };    // since C++20
+template <class T> using unwrap_reference_t = typename unwrap_reference<T>::type; // since C++20
+template <class T> using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type; // since C++20
+
 template <class T> // <class T=void> in C++14
 struct plus : binary_function<T, T, T>
 {
@@ -2527,6 +2532,14 @@ private:
 
 #endif // _LIBCPP_STD_VER > 14
 
+#if _LIBCPP_STD_VER > 17
+template <class _Tp>
+using unwrap_reference_t = typename unwrap_reference<_Tp>::type;
+
+template <class _Tp>
+using unwrap_ref_decay_t = typename unwrap_ref_decay<_Tp>::type;
+#endif // > C++17
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_FUNCTIONAL
diff --git a/libcxx/include/tuple b/libcxx/include/tuple
index 766f2e3c49b..2e54a5f6664 100644
--- a/libcxx/include/tuple
+++ b/libcxx/include/tuple
@@ -1078,30 +1078,12 @@ namespace {
   _LIBCPP_INLINE_VAR constexpr __ignore_t<unsigned char> ignore = __ignore_t<unsigned char>();
 }
 
-template <class _Tp>
-struct __make_tuple_return_impl
-{
-    typedef _Tp type;
-};
-
-template <class _Tp>
-struct __make_tuple_return_impl<reference_wrapper<_Tp> >
-{
-    typedef _Tp& type;
-};
-
-template <class _Tp>
-struct __make_tuple_return
-{
-    typedef typename __make_tuple_return_impl<typename decay<_Tp>::type>::type type;
-};
-
 template <class... _Tp>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
-tuple<typename __make_tuple_return<_Tp>::type...>
+tuple<typename __unwrap_ref_decay<_Tp>::type...>
 make_tuple(_Tp&&... __t)
 {
-    return tuple<typename __make_tuple_return<_Tp>::type...>(_VSTD::forward<_Tp>(__t)...);
+    return tuple<typename __unwrap_ref_decay<_Tp>::type...>(_VSTD::forward<_Tp>(__t)...);
 }
 
 template <class... _Tp>
diff --git a/libcxx/include/utility b/libcxx/include/utility
index 1dce49df4da..ddfa27e2223 100644
--- a/libcxx/include/utility
+++ b/libcxx/include/utility
@@ -616,32 +616,37 @@ swap(pair<_T1, _T2>& __x, pair<_T1, _T2>& __y)
     __x.swap(__y);
 }
 
-#ifndef _LIBCPP_CXX03_LANG
+template <class _Tp>
+struct __unwrap_reference { typedef _Tp type; };
 
 template <class _Tp>
-struct __make_pair_return_impl
-{
-    typedef _Tp type;
-};
+struct __unwrap_reference<reference_wrapper<_Tp> > { typedef _Tp& type; };
 
+#if _LIBCPP_STD_VER > 17
 template <class _Tp>
-struct __make_pair_return_impl<reference_wrapper<_Tp>>
-{
-    typedef _Tp& type;
-};
+struct unwrap_reference : __unwrap_reference<_Tp> { };
 
 template <class _Tp>
-struct __make_pair_return
-{
-    typedef typename __make_pair_return_impl<typename decay<_Tp>::type>::type type;
-};
+struct unwrap_ref_decay : unwrap_reference<typename decay<_Tp>::type> { };
+#endif // > C++17
+
+template <class _Tp>
+struct __unwrap_ref_decay
+#if _LIBCPP_STD_VER > 17
+    : unwrap_ref_decay<_Tp>
+#else
+    : __unwrap_reference<typename decay<_Tp>::type>
+#endif
+{ };
+
+#ifndef _LIBCPP_CXX03_LANG
 
 template <class _T1, class _T2>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
-pair<typename __make_pair_return<_T1>::type, typename __make_pair_return<_T2>::type>
+pair<typename __unwrap_ref_decay<_T1>::type, typename __unwrap_ref_decay<_T2>::type>
 make_pair(_T1&& __t1, _T2&& __t2)
 {
-    return pair<typename __make_pair_return<_T1>::type, typename __make_pair_return<_T2>::type>
+    return pair<typename __unwrap_ref_decay<_T1>::type, typename __unwrap_ref_decay<_T2>::type>
                (_VSTD::forward<_T1>(__t1), _VSTD::forward<_T2>(__t2));
 }
 
diff --git a/libcxx/test/std/utilities/function.objects/refwrap/unwrap_ref_decay.pass.cpp b/libcxx/test/std/utilities/function.objects/refwrap/unwrap_ref_decay.pass.cpp
new file mode 100644
index 00000000000..a63dc5b15f4
--- /dev/null
+++ b/libcxx/test/std/utilities/function.objects/refwrap/unwrap_ref_decay.pass.cpp
@@ -0,0 +1,58 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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_ref_decay;
+//
+// template <class T>
+// using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type;
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+
+#include <functional>
+#include <type_traits>
+
+
+template <typename T, typename Result>
+void check() {
+  static_assert(std::is_same_v<typename std::unwrap_ref_decay<T>::type, Result>);
+  static_assert(std::is_same_v<typename std::unwrap_ref_decay<T>::type, std::unwrap_ref_decay_t<T>>);
+}
+
+struct T { };
+
+int main() {
+  check<T,             T>();
+  check<T&,            T>();
+  check<T const,       T>();
+  check<T const&,      T>();
+  check<T*,            T*>();
+  check<T const*,      T const*>();
+  check<T[3],          T*>();
+  check<T const [3],   T const*>();
+  check<T (),          T (*)()>();
+  check<T (int) const, T (int) const>();
+  check<T (int) &,     T (int) &>();
+  check<T (int) &&,    T (int) &&>();
+
+  check<std::reference_wrapper<T>,         T&>();
+  check<std::reference_wrapper<T>&,        T&>();
+  check<std::reference_wrapper<T const>,   T const&>();
+  check<std::reference_wrapper<T const>&,  T const&>();
+  check<std::reference_wrapper<T*>,        T*&>();
+  check<std::reference_wrapper<T*>&,       T*&>();
+  check<std::reference_wrapper<T const*>,  T const*&>();
+  check<std::reference_wrapper<T const*>&, T const*&>();
+  check<std::reference_wrapper<T[3]>,      T (&)[3]>();
+  check<std::reference_wrapper<T[3]>&,     T (&)[3]>();
+  check<std::reference_wrapper<T ()>,      T (&)()>();
+  check<std::reference_wrapper<T ()>&,     T (&)()>();
+}
diff --git a/libcxx/test/std/utilities/function.objects/refwrap/unwrap_reference.pass.cpp b/libcxx/test/std/utilities/function.objects/refwrap/unwrap_reference.pass.cpp
new file mode 100644
index 00000000000..441ddf4bd56
--- /dev/null
+++ b/libcxx/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*>();
+}
diff --git a/libcxx/www/cxx2a_status.html b/libcxx/www/cxx2a_status.html
index 5b090a088a9..349d3c2bf34 100644
--- a/libcxx/www/cxx2a_status.html
+++ b/libcxx/www/cxx2a_status.html
@@ -108,7 +108,7 @@
 	<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/P0318R1">P0318R1</a></td><td>LWG</td><td>unwrap_ref_decay and unwrap_reference</td><td>San Diego</td><td>Complete</td><td>8.0</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/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>