It seems that Qt6 doesn't allow the type 'S' to act as a container for items of the type 'S'.

The following code fails to compile with Qt6 (but compiles with Qt5): 

struct S {
  using value_type = S; // causes compile error with Qt6
  int begin();
  int end();
};

Q_DECLARE_METATYPE(S)

Here are some compiler errors:

'value': is not a member of 'QTypeTraits::detail::expand_operator_equal_container<T,true>'

'value': is not a member of 'QTypeTraits::detail::expand_operator_equal_container<S::value_type,true>'

Similar messages about 'expand_operator_less_than_container' template are printed.

"Recursive" containers are sometimes useful. An example is nlohmann::json. However, changes introduced by Qt6 break applications that use such a types with the Qt property system (see also the issue description here)