Compare API dox of Q_DECLARE_METATYPE:
"
Some types are registered automatically and do not need this macro:
* Pointers to classes derived from QObject
* QPointer<T>, QSharedPointer<T>, QWeakPointer<T>,
where T is a class that derives from QObject
"
This is a ABI-compatible change, as this just replaces explicitly
defined template code (by the Q_DECLARE_METATYPE macro use) with the
generic predefined one picking up pointers to QObject subclasses.
So instead of the custom explicitly template overload
template <>
struct QMetaTypeId< TYPE >
{ /**/ static int qt_metatype_id() {/**/} };
the predefined template overload one is picked up via
template <typename T>
struct QMetaTypeId : public QMetaTypeIdQObject<T>
{ /**/ };
template <typename T, int =
QtPrivate::IsPointerToTypeDerivedFromQObject<T>::Value ?
QMetaType::PointerToQObject :
/**/ 0>
struct QMetaTypeIdQObject
{ /**/ };
template <typename T>
struct QMetaTypeIdQObject<T*, QMetaType::PointerToQObject>
{ /**/ static int qt_metatype_id() {/**/} };
And thus the constexpr code using QMetaTypeId<T>::qt_metatype_id(),
for which all this is done, evaluates as before.
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