[committed] libstdc++: Define C++23 std::forward_like (P2445R1)

diff --git a/libstdc++-v3/include/bits/move.h b/libstdc++-v3/include/bits/move.h
index 00997d6f1fb..0151d78aff9 100644
--- a/libstdc++-v3/include/bits/move.h
+++ b/libstdc++-v3/include/bits/move.h
@@ -89,6 +89,32 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return static_cast<_Tp&&>(__t);
     }
 
+#if __glibcxx_forward_like // C++ >= 23
+  template<typename _Tp, typename _Up>
+  [[nodiscard]]
+  constexpr decltype(auto)
+  forward_like(_Up&& __x) noexcept
+  {
+    constexpr bool __as_rval = is_rvalue_reference_v<_Tp&&>;
+
+    if constexpr (is_const_v<remove_reference_t<_Tp>>)
+      {
+	using _Up2 = remove_reference_t<_Up>;
+	if constexpr (__as_rval)
+	  return static_cast<const _Up2&&>(__x);
+	else
+	  return static_cast<const _Up2&>(__x);
+      }
+    else
+      {
+	if constexpr (__as_rval)
+	  return static_cast<remove_reference_t<_Up>&&>(__x);
+	else
+	  return static_cast<_Up&>(__x);
+      }
+  }
+#endif
+
   /**
    *  @brief  Convert a value to an rvalue.
    *  @param  __t  A thing of arbitrary type.
diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
index 6252f5478e0..8f008f9048f 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/libstdc++-v3/include/bits/version.def
@@ -1542,6 +1542,14 @@  ftms = {
   };
 };
 
+ftms = {
+  name = forward_like;
+  values = {
+    v = 202207;
+    cxxmin = 23;
+  };
+};
+
 ftms = {
   name = ios_noreplace;
   values = {
diff --git a/libstdc++-v3/include/std/utility b/libstdc++-v3/include/std/utility
index f30e802a88d..bdaf5d4c31b 100644
--- a/libstdc++-v3/include/std/utility
+++ b/libstdc++-v3/include/std/utility
@@ -68,9 +68,10 @@ 
 #include <bits/stl_relops.h>
 #include <bits/stl_pair.h>
 
-#define __glibcxx_want_exchange_function
-#define __glibcxx_want_constexpr_algorithms
 #define __glibcxx_want_as_const
+#define __glibcxx_want_constexpr_algorithms
+#define __glibcxx_want_exchange_function
+#define __glibcxx_want_forward_like
 #define __glibcxx_want_integer_comparison_functions
 #define __glibcxx_want_to_underlying
 #define __glibcxx_want_unreachable
diff --git a/libstdc++-v3/testsuite/20_util/forward_like/1.cc b/libstdc++-v3/testsuite/20_util/forward_like/1.cc
new file mode 100644
index 00000000000..928e60094a3
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/forward_like/1.cc
@@ -0,0 +1,59 @@ 
+// { dg-do compile { target c++23 } }
+// { dg-add-options no_pch }
+
+#include <utility>
+
+#ifndef __cpp_lib_forward_like
+# error "Feature-test macro for forward_like missing in <utility>"
+#elif __cpp_lib_forward_like != 202207L
+# error "Feature-test macro for forward_like has wrong value in <utility>"
+#endif
+
+template<typename T, typename U>
+using forward_like_t = decltype(std::forward_like<T>(std::declval<U>()));
+
+#if 0
+using std::is_same_v;
+#else
+#include <concepts>
+template<class T, class U> concept is_same_v = std::same_as<T, U>;
+#endif
+
+static_assert( is_same_v<forward_like_t<int, long>, long&&> );
+static_assert( is_same_v<forward_like_t<int&, long>, long&> );
+static_assert( is_same_v<forward_like_t<int&&, long>, long&&> );
+
+static_assert( is_same_v<forward_like_t<const int, long>, const long&&> );
+static_assert( is_same_v<forward_like_t<const int&, long>, const long&> );
+static_assert( is_same_v<forward_like_t<const int&&, long>, const long&&> );
+
+static_assert( is_same_v<forward_like_t<int, const long>, const long&&> );
+static_assert( is_same_v<forward_like_t<int&, const long>, const long&> );
+static_assert( is_same_v<forward_like_t<int&&, const long>, const long&&> );
+
+static_assert( is_same_v<forward_like_t<const int, long&>, const long&&> );
+static_assert( is_same_v<forward_like_t<const int&, long&>, const long&> );
+static_assert( is_same_v<forward_like_t<const int&&, long&>, const long&&> );
+
+static_assert( is_same_v<forward_like_t<int, const long&>, const long&&> );
+static_assert( is_same_v<forward_like_t<int&, const long&>, const long&> );
+static_assert( is_same_v<forward_like_t<int&&, const long&>, const long&&> );
+
+static_assert( is_same_v<forward_like_t<const int, long&&>, const long&&> );
+static_assert( is_same_v<forward_like_t<const int&, long&&>, const long&> );
+static_assert( is_same_v<forward_like_t<const int&&, long&&>, const long&&> );
+
+static_assert( is_same_v<forward_like_t<int, const long&&>, const long&&> );
+static_assert( is_same_v<forward_like_t<int&, const long&&>, const long&> );
+static_assert( is_same_v<forward_like_t<int&&, const long&&>, const long&&> );
+
+static_assert( is_same_v<forward_like_t<volatile int, long>, long&&> );
+static_assert( is_same_v<forward_like_t<volatile int&, long>, long&> );
+static_assert( is_same_v<forward_like_t<volatile int&&, long>, long&&> );
+
+static_assert( is_same_v<forward_like_t<const int, volatile long>,
+			 const volatile long&&> );
+static_assert( is_same_v<forward_like_t<const int&, volatile long>,
+			 const volatile long&> );
+static_assert( is_same_v<forward_like_t<const int&&, volatile long>,
+			 const volatile long&&> );
diff --git a/libstdc++-v3/testsuite/20_util/forward_like/2_neg.cc b/libstdc++-v3/testsuite/20_util/forward_like/2_neg.cc
new file mode 100644
index 00000000000..ff835af1915
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/forward_like/2_neg.cc
@@ -0,0 +1,10 @@ 
+// { dg-do compile { target c++23 } }
+
+#include <utility>
+
+auto x1 = std::forward_like<void>(1); // { dg-error "here" }
+// { dg-error "forming reference to void" "" { target *-*-* } 0 }
+auto x2 = std::forward_like<void()const>(1); // { dg-error "here" }
+// { dg-error "forming reference to qualified function" "" { target *-*-* } 0 }
+
+// { dg-prune-output "inconsistent deduction for auto return type" } // PR111484
diff --git a/libstdc++-v3/testsuite/20_util/forward_like/version.cc b/libstdc++-v3/testsuite/20_util/forward_like/version.cc
new file mode 100644
index 00000000000..058dccfabe2
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/forward_like/version.cc
@@ -0,0 +1,10 @@ 
+// { dg-do preprocess { target c++23 } }
+// { dg-add-options no_pch }
+
+#include <version>
+
+#ifndef __cpp_lib_forward_like
+# error "Feature-test macro for forward_like missing in <version>"
+#elif __cpp_lib_forward_like != 202207L
+# error "Feature-test macro for forward_like has wrong value in <version>"
+#endif