Fix QMarginsF::operator==() for zero margins

The (in)equality operators for QMarginsF are documented to use fuzzy
comparison. However, the implementation unconditionally used
qFuzzyCompare which is incorrect for the case of 0 margins.
Update the implementation to use a combination of qFuzzyIsNull and
qFuzzyCompare, like it's done for QPointF.

[ChangeLog][QtCore][QMarginsF] Fixed a bug when equality comparison
returned incorrect results if one of the margins was zero.

Task-number: QTBUG-120308
Change-Id: I6e785fe8b523f6aa5f5317fb75877fdbf4e086c2
Reviewed-by: Tatiana Borisova <tatiana.borisova@qt.io>

src/corelib/tools/qmargins.h

@@ -304,18 +304,25 @@ private:
     qreal m_right;
     qreal m_bottom;
 
+    QT_WARNING_PUSH
+    QT_WARNING_DISABLE_FLOAT_COMPARE
     friend constexpr inline bool operator==(const QMarginsF &lhs, const QMarginsF &rhs) noexcept
     {
-        return qFuzzyCompare(lhs.left(), rhs.left())
+        return ((!lhs.m_left || !rhs.m_left) ? qFuzzyIsNull(lhs.m_left - rhs.m_left)
-            && qFuzzyCompare(lhs.top(), rhs.top())
+                                             : qFuzzyCompare(lhs.m_left, rhs.m_left))
-            && qFuzzyCompare(lhs.right(), rhs.right())
+            && ((!lhs.m_top || !rhs.m_top) ? qFuzzyIsNull(lhs.m_top - rhs.m_top)
-            && qFuzzyCompare(lhs.bottom(), rhs.bottom());
+                                           : qFuzzyCompare(lhs.m_top, rhs.m_top))
+            && ((!lhs.m_right || !rhs.m_right) ? qFuzzyIsNull(lhs.m_right - rhs.m_right)
+                                               : qFuzzyCompare(lhs.m_right, rhs.m_right))
+            && ((!lhs.m_bottom || !rhs.m_bottom) ? qFuzzyIsNull(lhs.m_bottom - rhs.m_bottom)
+                                                 : qFuzzyCompare(lhs.m_bottom, rhs.m_bottom));
     }
 
     friend constexpr inline bool operator!=(const QMarginsF &lhs, const QMarginsF &rhs) noexcept
     {
         return !(lhs == rhs);
     }
+    QT_WARNING_POP
 
     template <std::size_t I,
               typename M,

tests/auto/corelib/tools/qmargins/CMakeLists.txt

@@ -14,4 +14,6 @@ endif()
 qt_internal_add_test(tst_qmargins
     SOURCES
         tst_qmargins.cpp
+    LIBRARIES
+        Qt::TestPrivate
 )

tests/auto/corelib/tools/qmargins/tst_qmargins.cpp

@@ -32,15 +32,21 @@ CHECK(const &&);
 
 #include <QTest>
 #include <qmargins.h>
+#include <private/qcomparisontesthelper_p.h>
 
 #include <array>
 
 Q_DECLARE_METATYPE(QMargins)
 
+constexpr static qreal qreal_min = std::numeric_limits<qreal>::min();
+
 class tst_QMargins : public QObject
 {
     Q_OBJECT
 private slots:
+    void comparison_data();
+    void comparison();
+
     void getSetCheck();
 #ifndef QT_NO_DATASTREAM
     void dataStreamCheck();
@@ -65,6 +71,46 @@ private slots:
     void toMarginsF();
 };
 
+void tst_QMargins::comparison_data()
+{
+    QTest::addColumn<QMarginsF>("lhs");
+    QTest::addColumn<QMarginsF>("rhs");
+    QTest::addColumn<bool>("result");
+    QTest::addColumn<bool>("floatResult");
+    QTest::addColumn<bool>("mixedResult");
+
+    auto row = [](const QMarginsF &lhs, const QMarginsF &rhs, bool res, bool fRes, bool mRes) {
+        QString str;
+        QDebug dbg(&str);
+        dbg.nospace() << "(" << lhs.left() << ", " << lhs.top() << ", " << lhs.right() << ", "
+                      << lhs.bottom() << ") vs (" << rhs.left() << ", " << rhs.top() << ", "
+                      << rhs.right() << ", " << rhs.bottom() << ")";
+        QTest::addRow("%s", str.toLatin1().constData()) << lhs << rhs << res << fRes << mRes;
+    };
+
+    row(QMarginsF(0.0, 0.0, 0.0, 0.0), QMarginsF(0.0, 0.0, 0.0, 0.0), true, true, true);
+    row(QMarginsF(qreal_min, -qreal_min, -qreal_min, qreal_min), QMarginsF(0.0, 0.0, 0.0, 0.0), true, true, true);
+    row(QMarginsF(1.0, 2.0, 3.0, 4.0), QMarginsF(1.1, 2.1, 2.9, 3.9), true, false, true);
+    row(QMarginsF(1.5, 2.5, 3.0, 4.0), QMarginsF(1.1, 2.1, 2.9, 3.9), false, false, false);
+}
+
+void tst_QMargins::comparison()
+{
+    QFETCH(const QMarginsF, lhs);
+    QFETCH(const QMarginsF, rhs);
+    QFETCH(const bool, result);
+    QFETCH(const bool, floatResult);
+    QFETCH(const bool, mixedResult);
+
+    QT_TEST_EQUALITY_OPS(lhs, rhs, floatResult);
+
+    const QMargins lhsInt = lhs.toMargins();
+    const QMargins rhsInt = rhs.toMargins();
+    QT_TEST_EQUALITY_OPS(lhsInt, rhsInt, result);
+
+    QT_TEST_EQUALITY_OPS(lhs, rhsInt, mixedResult);
+}
+
 // Testing get/set functions
 void tst_QMargins::getSetCheck()
 {