Fix asymmetry in QVariant::cmp when A converts to B but not the opposite

This would result in (a == b) != (b == a). The == operation should be
commutative as much as possible.

Now, there's still an asymmetry in that b is forced to a type and the
conversion may fail. QVariant should have an idea of what conversions
are "promotion" and which ones are "demotion" (subject to loss of data
and/or can fail), so it can do the promotion first

Task-number: QTBUG-42254
Change-Id: I9fa4496bbbf0f8719ff8456cc24247290beac608
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>

2 files changed, 17 insertions, 3 deletions

diff --git a/src/corelib/kernel/qvariant.cpp b/src/corelib/kernel/qvariant.cpp
index 73a951c39b..67911f2836 100644
--- a/src/corelib/kernel/qvariant.cpp
+++ b/src/corelib/kernel/qvariant.cpp
@@ -3161,8 +3161,15 @@ bool QVariant::cmp(const QVariant &v) const
             else
                 return toLongLong() == v.toLongLong();
         }
-        if (!v2.canConvert(v1.d.type) || !v2.convert(v1.d.type))
-            return false;
+        if (v2.canConvert(v1.d.type)) {
+            if (!v2.convert(v1.d.type))
+                return false;
+        } else {
+            // try the opposite conversion, it might work
+            qSwap(v1, v2);
+            if (!v2.convert(v1.d.type))
+                return false;
+        }
     }
     if (v1.d.type >= QMetaType::User) {
         int result;
diff --git a/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp b/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
index 10c7e425f0..eae6311854 100644
--- a/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
+++ b/tests/auto/corelib/kernel/qvariant/tst_qvariant.cpp
@@ -43,7 +43,7 @@
 #include <qlocale.h>
 #include <qdebug.h>
 #include <qjsondocument.h>
-
+#include <quuid.h>
 
 #include <limits.h>
 #include <float.h>
@@ -1437,6 +1437,13 @@ void tst_QVariant::operator_eq_eq_data()
     // ### many other combinations missing
 
     {
+        // QUuid can convert to QString, but not the opposite
+        QUuid uuid(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
+        QTest::newRow("uuidstring") << QVariant(uuid) << QVariant(uuid.toString()) << true;
+        QTest::newRow("stringuuid") << QVariant(uuid.toString()) << QVariant(uuid) << true;
+    }
+
+    {
         QMap<QString, QVariant> map1;
         map1.insert( "X", 1 );