QMetaType: fix QMetaTypes for non-const references

Namely, they shouldn't be supported. Even trying to create such a type
(as in QMetaType::fromType<int &>()) should fail, because for the
purposes of the meta type, they are not the same.

However, they were being registered in the meta objects' meta type list
as a mistake since commit cb43aaca112c864a201b87037692cb8ae2e93b6d
("Introduce QMetaObject::metaType"), including for output parameters in
D-Bus remote objects' meta objects. despite the comment saying "type id
not available".

[ChangeLog][Potentially Source-incompatible Changes] Made meta types for
non-const references fail to compile. Previously, QMetaType::fromType
allowed this to compile, but returned the meta type for the base type,
which was incorrect. Const references are understood to be the same as
the base type.

[ChangeLog][Important Behavior Changes] The meta type for non-const
reference parameters in extracted methods (signals, slots, etc.) is no
longer available in QMetaMethod. This used to be the case in Qt 4.x and
5.x, but due to a mistake in Qt 6.0-6.3, QMetaMethod would incorrectly
report the base (non-reference) type. Additionally, both the reference
and the non-reference types may have been reported in different APIs.

Pick-to: 6.4
Change-Id: I36b24183fbd041179f2ffffd1702384d2b64a5f9
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

1 files changed, 19 insertions, 9 deletions

diff --git a/src/corelib/kernel/qmetatype.h b/src/corelib/kernel/qmetatype.h
index 18d59e1298..ae13590658 100644
--- a/src/corelib/kernel/qmetatype.h
+++ b/src/corelib/kernel/qmetatype.h
@@ -843,7 +843,11 @@ namespace QtPrivate
     } // namespace detail
 
     template <typename T, typename ODR_VIOLATION_PREVENTER>
-    struct is_complete : detail::is_complete_helper<T, ODR_VIOLATION_PREVENTER>::type {};
+    struct is_complete : detail::is_complete_helper<std::remove_reference_t<T>, ODR_VIOLATION_PREVENTER>::type {};
+
+    template <typename T> struct MetatypeDecay              { using type = T; };
+    template <typename T> struct MetatypeDecay<const T>     { using type = T; };
+    template <typename T> struct MetatypeDecay<const T &>   { using type = T; };
 
     template<typename T>
     struct IsPointerToTypeDerivedFromQObject
@@ -2478,12 +2482,6 @@ QT_FOR_EACH_STATIC_CORE_POINTER(QT_METATYPE_DECLARE_EXTERN_TEMPLATE_ITER)
 QT_FOR_EACH_STATIC_CORE_TEMPLATE(QT_METATYPE_DECLARE_EXTERN_TEMPLATE_ITER)
 #undef QT_METATYPE_DECLARE_EXTERN_TEMPLATE_ITER
 #endif
-template<typename T>
-constexpr const QMetaTypeInterface *qMetaTypeInterfaceForType()
-{
-    using Ty = std::remove_cv_t<std::remove_reference_t<T>>;
-    return &QMetaTypeInterfaceWrapper<Ty>::metaType;
-}
 
 template<typename T>
 struct qRemovePointerLike
@@ -2510,14 +2508,26 @@ struct TypeAndForceComplete
     using ForceComplete = ForceComplete_;
 };
 
+template<typename T>
+constexpr const QMetaTypeInterface *qMetaTypeInterfaceForType()
+{
+    using Ty = typename MetatypeDecay<T>::type;
+    return &QMetaTypeInterfaceWrapper<Ty>::metaType;
+}
+
 template<typename Unique, typename TypeCompletePair>
 constexpr const QMetaTypeInterface *qTryMetaTypeInterfaceForType()
 {
     using T = typename TypeCompletePair::type;
     using ForceComplete = typename TypeCompletePair::ForceComplete;
-    using Ty = std::remove_cv_t<std::remove_reference_t<T>>;
+    using Ty = typename MetatypeDecay<T>::type;
     using Tz = qRemovePointerLike_t<Ty>;
-    if constexpr (!is_complete<Tz, Unique>::value && !ForceComplete::value) {
+
+    if constexpr (ForceComplete::value) {
+        return &QMetaTypeInterfaceWrapper<Ty>::metaType;
+    } else if constexpr (std::is_reference_v<Tz>) {
+        return nullptr;
+    } else if constexpr (!is_complete<Tz, Unique>::value) {
         return nullptr;
     } else {
         return &QMetaTypeInterfaceWrapper<Ty>::metaType;