Short live namespace q20!

As proposed already for Qt 5 (then with q14 and q17), add
reimplementations of C++20 library features into a q20 namespace. The
advantage is that we can later just s/q20::/std::/ and be sure that a)
the q20 functionality works (within reason) like the std counterparts
and b) we don't have to re-indent the code after the replacement.

Start with std::identity in q20functional.h, required by
std::ranges::{any,all,none}_of() (ex ranges[1]) in q20algorithm.h,
which I happen to require in QLibrary next.

[1] We can't provide the nice range-based overloads (any_of(vector,
pred)), yet, because that would require to reimplement all the
range-related concepts, as the overloads are ambiguous if
unconstrained. First, we should check whether we can't just depend on
Nieber's ranges-v3 library instead. The q20 namespace is independent
of this decision, as it's more universally useful (q20::remove_cvref
comes to mind).

Unlike in q14/q17 times, a single header, q20.h, would become unwieldy
very soon, so use separate headers.

Change-Id: I14b98c865e242d2dc9674bc1bff5f7a9e4db9940
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 196 insertions, 0 deletions

diff --git a/src/corelib/global/q20algorithm.h b/src/corelib/global/q20algorithm.h
new file mode 100644
index 0000000000..d8e5f51fa0
--- /dev/null
+++ b/src/corelib/global/q20algorithm.h
@@ -0,0 +1,116 @@
+/****************************************************************************
+**
+** Copyright (C) 2021 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Marc Mutz <marc.mutz@kdab.com>
+** Contact: http://www.qt.io/licensing/
+**
+** This file is part of the QtCore module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+#ifndef Q20ALGORITHM_H
+#define Q20ALGORITHM_H
+
+#include <QtCore/qglobal.h>
+
+#include <algorithm>
+#include <QtCore/q20functional.h>
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API. Types and functions defined
+// in this file will behave exactly as their std counterparts. 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
+// definitons will JUST go away.
+//
+// If you can't agree to these terms, don't use these definitons!
+//
+// We mean it.
+//
+
+QT_BEGIN_NAMESPACE
+
+namespace q20::ranges {
+// like std::ranges::{any,all,none}_of, just unconstrained, so no range-overload
+#ifdef __cpp_lib_ranges
+using std::ranges::any_of;
+using std::ranges::all_of;
+using std::ranges::none_of;
+#else
+[[maybe_unused]] inline constexpr struct { // Niebloid
+    template <typename InputIterator, typename Sentinel,
+              typename Predicate, typename Projection = q20::identity>
+    constexpr bool operator()(InputIterator first, Sentinel last, Predicate pred, Projection proj = {}) const
+    {
+        while (first != last) {
+            if (std::invoke(pred, std::invoke(proj, *first)))
+                return true;
+            ++first;
+        }
+        return false;
+    }
+} any_of;
+[[maybe_unused]] inline constexpr struct { // Niebloid
+    template <typename InputIterator, typename Sentinel,
+              typename Predicate, typename Projection = q20::identity>
+    constexpr bool operator()(InputIterator first, Sentinel last, Predicate pred, Projection proj = {}) const
+    {
+        while (first != last) {
+            if (!std::invoke(pred, std::invoke(proj, *first)))
+                return false;
+            ++first;
+        }
+        return true;
+    }
+} all_of;
+[[maybe_unused]] inline constexpr struct { // Niebloid
+    template <typename InputIterator, typename Sentinel,
+              typename Predicate, typename Projection = q20::identity>
+    constexpr bool operator()(InputIterator first, Sentinel last, Predicate pred, Projection proj = {}) const
+    {
+        while (first != last) {
+            if (std::invoke(pred, std::invoke(proj, *first)))
+                return false;
+            ++first;
+        }
+        return true;
+    }
+} none_of;
+#endif // __cpp_lib_ranges
+} // namespace q20::ranges
+
+QT_END_NAMESPACE
+
+#endif /* Q20ALGORITHM_H */
diff --git a/src/corelib/global/q20functional.h b/src/corelib/global/q20functional.h
new file mode 100644
index 0000000000..f35614f837
--- /dev/null
+++ b/src/corelib/global/q20functional.h
@@ -0,0 +1,80 @@
+/****************************************************************************
+**
+** Copyright (C) 2021 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Marc Mutz <marc.mutz@kdab.com>
+** Contact: http://www.qt.io/licensing/
+**
+** This file is part of the QtCore module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+#ifndef Q20FUNCTIONAL_H
+#define Q20FUNCTIONAL_H
+
+#include <QtCore/qglobal.h>
+
+#include <functional>
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API. Types and functions defined
+// in this file will behave exactly as their std counterparts. 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
+// definitons will JUST go away.
+//
+// If you can't agree to these terms, don't use these definitons!
+//
+// We mean it.
+//
+
+QT_BEGIN_NAMESPACE
+
+namespace q20 {
+// like std::identity
+#ifdef __cpp_lib_ranges
+using std::identity;
+#else
+struct identity
+{
+    struct is_transparent {};
+    template <typename T>
+    constexpr T &&operator()(T&& t) const noexcept { return std::forward<T>(t); }
+};
+#endif // __cpp_lib_ranges
+} // namespace q20
+
+QT_END_NAMESPACE
+
+#endif /* Q20FUNCTIONAL_H */