QQmlProperty: Accept badly capitalized signal handler names

... but warn about them. Apparently we did accept them prior to 6.4.

Fixes: QTBUG-116576
Change-Id: If890db85f5a8d71c0bcdfaf646ee9f01765a0b3c
Reviewed-by: Sami Shalayel <sami.shalayel@qt.io>
(cherry picked from commit 2fc19f2c627802d26bd385a6c9cc1f6412852ee2)

3 files changed, 59 insertions, 0 deletions

diff --git a/src/qml/qml/qqmlproperty.cpp b/src/qml/qml/qqmlproperty.cpp
index 0e44df219f..ba6a0971f2 100644
--- a/src/qml/qml/qqmlproperty.cpp
+++ b/src/qml/qml/qqmlproperty.cpp
@@ -380,6 +380,9 @@ void QQmlPropertyPrivate::initProperty(QObject *obj, const QString &name,
         return false;
     };
 
+    static constexpr QLatin1String On("on");
+    static constexpr qsizetype StrlenOn = On.size();
+
     const QString terminalString = terminal.toString();
     if (QmlIR::IRBuilder::isSignalPropertyName(terminalString)) {
         QString signalName = terminalString.mid(2);
@@ -412,6 +415,26 @@ void QQmlPropertyPrivate::initProperty(QObject *obj, const QString &name,
         } else if (findSignalInMetaObject(signalName.toUtf8())) {
             return;
         }
+    } else if (terminalString.size() > StrlenOn && terminalString.startsWith(On)) {
+        // This is quite wrong. But we need it for backwards compatibility.
+        QString signalName = terminalString.sliced(2);
+        signalName.front() = signalName.front().toLower();
+
+        QString handlerName = On + signalName;
+        const auto end = handlerName.end();
+        auto result = std::find_if(
+                std::next(handlerName.begin(), StrlenOn), end,
+                [](const QChar &c) { return c.isLetter(); });
+        if (result != end)
+            *result = result->toUpper();
+
+        qWarning()
+                << terminalString
+                << "is not a properly capitalized signal handler name."
+                << handlerName
+                << "would be correct.";
+        if (findSignalInMetaObject(signalName.toUtf8()))
+            return;
     }
 
     if (ddata && ddata->propertyCache) {
diff --git a/tests/auto/qml/qqmlconnections/data/badSignalHandlerName.qml b/tests/auto/qml/qqmlconnections/data/badSignalHandlerName.qml
new file mode 100644
index 0000000000..921787aa36
--- /dev/null
+++ b/tests/auto/qml/qqmlconnections/data/badSignalHandlerName.qml
@@ -0,0 +1,15 @@
+import QtQml
+
+QtObject {
+    id: root
+    signal _foo
+
+    property int handled: 0
+
+    property Connections c: Connections {
+        target: root
+        function on_Foo() { root.handled += 1 }
+        function on_foo() { root.handled += 2 }
+    }
+}
+
diff --git a/tests/auto/qml/qqmlconnections/tst_qqmlconnections.cpp b/tests/auto/qml/qqmlconnections/tst_qqmlconnections.cpp
index f23c474907..63dfe9a3db 100644
--- a/tests/auto/qml/qqmlconnections/tst_qqmlconnections.cpp
+++ b/tests/auto/qml/qqmlconnections/tst_qqmlconnections.cpp
@@ -55,6 +55,7 @@ private slots:
     void bindToPropertyWithUnderscoreChangeHandler();
     void invalidTarget();
 
+    void badSignalHandlerName();
 private:
     QQmlEngine engine;
     void prefixes();
@@ -485,6 +486,26 @@ void tst_qqmlconnections::invalidTarget()
     QTRY_VERIFY(root->objectName().isEmpty());
 }
 
+void tst_qqmlconnections::badSignalHandlerName()
+{
+    QQmlEngine engine;
+    QQmlComponent component(&engine, testFileUrl("badSignalHandlerName.qml"));
+    QVERIFY2(component.isReady(), qPrintable(component.errorString()));
+
+    QTest::ignoreMessage(
+            QtWarningMsg,
+            "\"on_foo\" is not a properly capitalized signal handler name. "
+            "\"on_Foo\" would be correct.");
+
+    QScopedPointer<QObject> root(component.create());
+    QVERIFY(!root.isNull());
+
+    QCOMPARE(root->property("handled").toInt(), 0);
+    QMetaObject::invokeMethod(root.data(), "_foo");
+    QCOMPARE(root->property("handled").toInt(), 3);
+}
+
+
 QTEST_MAIN(tst_qqmlconnections)
 
 #include "tst_qqmlconnections.moc"