Short live q23::forward_like!

Following https://eel.is/c++draft/forward#lib:forward_like
step-by-step.

Temporarily add a q23utility.cpp to contain the static_assert()
tests. There's no point in establishing a tst_q23utility just to run
six static assertions. Should remove the .cpp again once an in-tree
user exercises the function with the same coverage.

Task-number: QTBUG-111598
Change-Id: Icb18eae089d65ca4e555921dbb71e74a7e255645
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/CMakeLists.txt

@@ -104,6 +104,8 @@ qt_internal_add_module(Core
         global/q20memory.h
         global/q20type_traits.h
         global/q23functional.h
+        global/q23utility.cpp # remove once we have a user that tests this
+        global/q23utility.h
         global/qxpfunctional.h
         global/qxptype_traits.h
         ipc/qsharedmemory.cpp ipc/qsharedmemory.h ipc/qsharedmemory_p.h

src/corelib/global/q23utility.cpp

@@ -0,0 +1,25 @@
+// Copyright (C) 2023 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
+
+#include <QtCore/q23utility.h>
+
+QT_BEGIN_NAMESPACE
+
+#define CHECK2(cvref_in, cvref_out) \
+    static_assert(std::is_same_v< \
+            decltype(q23::forward_like<int cvref_in >(std::declval<long&>())), \
+            long cvref_out \
+        >, "oops: cvref '" #cvref_in "' doesn't work") \
+    /* end */
+#define CHECK(cvref) CHECK2(cvref, cvref)
+CHECK2(/**/, &&);
+CHECK(&);
+CHECK(&&);
+CHECK2(const, const &&);
+CHECK(const &);
+CHECK(const &&);
+// volatile is not supported
+#undef CHECK
+#undef CHECK2
+
+QT_END_NAMESPACE

src/corelib/global/q23utility.h

@@ -0,0 +1,75 @@
+// Copyright (C) 2023 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
+#ifndef Q23UTILITY_H
+#define Q23UTILITY_H
+
+#include <QtCore/qtconfigmacros.h>
+
+#include <utility>
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API. Types and functions defined in this
+// file can reliably be replaced by their std counterparts, once available.
+// You may use these definitions in your own code, but be aware that we
+// will remove them once Qt depends on the C++ version that supports
+// them in namespace std. There will be NO deprecation warning, the
+// definitions will JUST go away.
+//
+// If you can't agree to these terms, don't use these definitions!
+//
+// We mean it.
+//
+
+QT_BEGIN_NAMESPACE
+
+namespace q23 {
+// like std::forward_like
+#ifdef __cpp_lib_forward_like
+using std::forward_like;
+#else
+
+namespace _detail {
+
+// [forward]/6.1 COPY_CONST
+template <typename A, typename B>
+using copy_const_t = std::conditional_t<
+        std::is_const_v<A>, const B,
+        /* else */          B
+    >;
+
+// [forward]/6.2 OVERRIDE_REF
+template <typename A, typename B>
+using override_ref_t = std::conditional_t<
+        std::is_rvalue_reference_v<A>, std::remove_reference_t<B>&&,
+        /* else */                     B&
+    >;
+
+// [forward]/6.3 "V"
+template <typename T, typename U>
+using forward_like_ret_t = override_ref_t<
+        T&&,
+        copy_const_t<
+            std::remove_reference_t<T>,
+            std::remove_reference_t<U>
+        >
+    >;
+
+} // namespace detail
+
+// http://eel.is/c++draft/forward#lib:forward_like
+template <class T, class U>
+[[nodiscard]] constexpr auto forward_like(U &&x) noexcept
+    -> _detail::forward_like_ret_t<T, U>
+{
+    using V = _detail::forward_like_ret_t<T, U>;
+    return static_cast<V>(x);
+}
+#endif // __cpp_lib_forward_like
+} // namespace q23
+
+QT_END_NAMESPACE
+
+#endif /* Q23UTILITY_H */