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diff --git a/src/corelib/tools/qduplicatetracker_p.h b/src/corelib/tools/qduplicatetracker_p.h
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+/****************************************************************************
+**
+** Copyright (C) 2020 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 QDUPLICATETRACKER_P_H
+#define QDUPLICATETRACKER_P_H
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API.  It exists purely as an
+// implementation detail.  This header file may change from version to
+// version without notice, or even be removed.
+//
+// We mean it.
+//
+
+#include <qglobal.h>
+
+#if QT_HAS_INCLUDE(<memory_resource>) && __cplusplus > 201402L
+#  include <unordered_set>
+#  include <memory_resource>
+#else
+#  include <qset.h>
+#endif
+
+QT_BEGIN_NAMESPACE
+
+template <typename T, size_t Prealloc = 32>
+class QDuplicateTracker {
+#ifdef __cpp_lib_memory_resource
+    char buffer[Prealloc * sizeof(T)];
+    std::pmr::monotonic_buffer_resource res{buffer, sizeof buffer};
+    std::pmr::unordered_set<T> set{&res};
+#else
+    QSet<T> set;
+    int setSize = 0;
+#endif
+    Q_DISABLE_COPY_MOVE(QDuplicateTracker);
+public:
+    QDuplicateTracker() = default;
+    void reserve(int n) { set.reserve(n); }
+    Q_REQUIRED_RESULT bool hasSeen(const T &s)
+    {
+        bool inserted;
+#ifdef __cpp_lib_memory_resource
+        inserted = set.insert(s).second;
+#else
+        set.insert(s);
+        const int n = set.size();
+        inserted = qExchange(setSize, n) != n;
+#endif
+        return !inserted;
+    }
+};
+
+QT_END_NAMESPACE
+
+#endif /* QDUPLICATETRACKER_P_H */