QJSEngine: Fix some conversions

The optimization for object-to-string conversion would never trigger
because we were checking the wrong metatype for the PointerToQObject
flag.

Furthermore, we can provide a very simple optimization for the case when
we just want a QObject*. Finally, if we have both types, we can use
is_base_of_v to optimize cases where we are converting to a base class.

Pick-to: 6.5
Task-number: QTBUG-111986
Change-Id: I731fe0398730a2a83222d8c2fdff8aa3d21f7aec
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

1 files changed, 11 insertions, 2 deletions

diff --git a/src/qml/jsapi/qjsengine.h b/src/qml/jsapi/qjsengine.h
index c04e47d34e..dbeecb45ae 100644
--- a/src/qml/jsapi/qjsengine.h
+++ b/src/qml/jsapi/qjsengine.h
@@ -139,12 +139,21 @@ public:
             return toPrimitiveValue(value);
 
         if constexpr (std::is_same_v<T, QString>) {
-            if (targetType.flags() & QMetaType::PointerToQObject) {
+            if (sourceType.flags() & QMetaType::PointerToQObject) {
                 return convertQObjectToString(
                             *reinterpret_cast<QObject *const *>(value.constData()));
             }
         }
 
+        if constexpr (std::is_same_v<QObject, std::remove_const_t<std::remove_pointer_t<T>>>) {
+            if (sourceType.flags() & QMetaType::PointerToQObject) {
+                return *static_cast<QObject *const *>(value.constData());
+
+                // We should not access source->metaObject() here since that may trigger some
+                // rather involved code. convertVariant() can do this using property caches.
+            }
+        }
+
         if (sourceType == QMetaType::fromType<QJSValue>())
             return fromScriptValue<T>(*reinterpret_cast<const QJSValue *>(value.constData()));
 
@@ -175,7 +184,7 @@ public:
     template<typename From, typename To>
     inline To coerceValue(const From &from)
     {
-        if constexpr (std::is_same_v<From, To>)
+        if constexpr (std::is_base_of_v<To, From>)
             return from;
 
         if constexpr (std::is_same_v<To, QJSValue>)