QPainterStateGuard: make movable, add test coverage

There's no real reason not to be able to move a state guard.

Since a moved-from QPainterStateGuard must not have a restore-
level count, we have to implement the move-constructor explicily.
Add a test case, which verifies that we don't end up with an
un-balanced save/restore count, and that verifies that we would
trigger the Q_ASSERT if we do.

Address comment from header review,
amends 9ecf47a8a8d11227ecf192246d7df7c2c4dc9105.

Task-number: QTBUG-132090
Change-Id: I1db135bf48c0fa0a7bac4fdae7b7263c356b5eb6
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>
(cherry picked from commit ec3a5f4994a2bafc65fa8e01fb0861219580f622)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/painting/qpainterstateguard.cpp

@@ -37,6 +37,25 @@ QT_BEGIN_NAMESPACE
     QPainter's state.
 */
 
+/*!
+    \fn QPainterStateGuard::QPainterStateGuard(QPainterStateGuard &&other)
+
+    Move-constructs a painter state guard from \a other.
+*/
+
+/*!
+    \fn QPainterStateGuard &QPainterStateGuard::operator=(QPainterStateGuard &&other)
+
+    Move-assigns \a other to this painter state guard.
+*/
+
+/*!
+    \fn void QPainterStateGuard::swap(QPainterStateGuard &other)
+
+    Swaps the \a other with this painter state guard. This operation is very
+    fast and never fails.
+*/
+
 /*!
     \fn QPainterStateGuard::~QPainterStateGuard()
     Destroys the QPainterStateGuard instance and calls restore() as often as save()

src/gui/painting/qpainterstateguard.h

@@ -11,13 +11,24 @@ QT_BEGIN_NAMESPACE
 
 class QPainterStateGuard
 {
-    Q_DISABLE_COPY_MOVE(QPainterStateGuard)
+    Q_DISABLE_COPY(QPainterStateGuard)
 public:
     enum class InitialState : quint8 {
         Save,
         NoSave,
     };
 
+    QPainterStateGuard(QPainterStateGuard &&other) noexcept
+        : m_painter(std::exchange(other.m_painter, nullptr))
+        , m_level(std::exchange(other.m_level, 0))
+    {}
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_MOVE_AND_SWAP(QPainterStateGuard)
+    void swap(QPainterStateGuard &other) noexcept
+    {
+        qt_ptr_swap(m_painter, other.m_painter);
+        std::swap(m_level, other.m_level);
+    }
+
     Q_NODISCARD_CTOR
     explicit QPainterStateGuard(QPainter *painter, InitialState state = InitialState::Save)
         : m_painter(painter)

tests/auto/gui/painting/CMakeLists.txt

@@ -10,6 +10,7 @@ add_subdirectory(qpagelayout)
 add_subdirectory(qpageranges)
 add_subdirectory(qpagesize)
 add_subdirectory(qpainter)
+add_subdirectory(qpainterstateguard)
 if (QT_FEATURE_pdf)
     add_subdirectory(qpdfwriter)
 endif()

tests/auto/gui/painting/qpainterstateguard/CMakeLists.txt

@@ -0,0 +1,19 @@
+# Copyright (C) 2024 The Qt Company Ltd.
+# SPDX-License-Identifier: BSD-3-Clause
+
+#####################################################################
+## tst_qpainterstateguard Test:
+#####################################################################
+
+if(NOT QT_BUILD_STANDALONE_TESTS AND NOT QT_BUILDING_QT)
+    cmake_minimum_required(VERSION 3.16)
+    project(tst_qpainterstateguard LANGUAGES CXX)
+    find_package(Qt6BuildInternals REQUIRED COMPONENTS STANDALONE_TEST)
+endif()
+
+qt_internal_add_test(tst_qpainterstateguard
+    SOURCES
+        tst_qpainterstateguard.cpp
+    LIBRARIES
+        Qt::Gui
+)

tests/auto/gui/painting/qpainterstateguard/tst_qpainterstateguard.cpp

@@ -0,0 +1,69 @@
+// Copyright (C) 2024 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only
+
+#define Q_ASSERT(cond) ((cond) ? static_cast<void>(0) : qCritical(#cond))
+
+#include <QTest>
+
+#include <QtGui/qpixmap.h>
+#include <QtGui/qpainter.h>
+#include <QtGui/qpainterstateguard.h>
+
+class tst_QPainterStateGuard : public QObject
+{
+    Q_OBJECT
+
+private slots:
+    void basics_data();
+    void basics();
+};
+
+void tst_QPainterStateGuard::basics_data()
+{
+    QTest::addColumn<QPainterStateGuard::InitialState>("initialState");
+    QTest::addColumn<bool>("expectWarning");
+
+    QTest::addRow("Save") << QPainterStateGuard::InitialState::Save << false;
+    QTest::addRow("NoSave") << QPainterStateGuard::InitialState::NoSave << true;
+}
+
+void tst_QPainterStateGuard::basics()
+{
+    QFETCH(const QPainterStateGuard::InitialState, initialState);
+    QFETCH(const bool, expectWarning);
+
+    if (expectWarning) {
+        QTest::ignoreMessage(QtCriticalMsg, "m_level > 0");
+        QTest::ignoreMessage(QtWarningMsg, "QPainter::restore: Unbalanced save/restore");
+    } else {
+        QTest::failOnWarning("QPainter::restore: Unbalanced save/restore");
+    }
+    QPixmap pixmap(100, 100);
+    QPainter painter(&pixmap);
+    const QPen defaultPen = painter.pen();
+
+    {
+        auto makeGuard = [initialState](QPainter *p) -> QPainterStateGuard {
+            return QPainterStateGuard(p, initialState);
+        };
+        QPainterStateGuard guard = makeGuard(&painter);
+
+        painter.setPen(Qt::red);
+        QCOMPARE(painter.pen().color(), Qt::red);
+
+        QPainterStateGuard guard2 = std::move(guard);
+
+        QCOMPARE(painter.pen().color(), Qt::red);
+
+        guard2.restore();
+    }
+
+    if (expectWarning)
+        QCOMPARE_NE(painter.pen(), defaultPen);
+    else
+        QCOMPARE(painter.pen(), defaultPen);
+}
+
+QTEST_MAIN(tst_QPainterStateGuard)
+
+#include "tst_qpainterstateguard.moc"