From andrei's notes:

• \terra incognita
• they define a new type (as if having a separate QML file), so must be handled similarly (extra care should be given to how context hierarchy is handled for them: as for document roots?)
• they use the compilation unit of the QML file (no separate CU)
• they support cyclic references but not cyclic instantiations (would it actually work for qmltc? might have to use QQmlComponent wrapping just because of this)

  import QtQuick 2.15
  Item {
    component A : Item {
      property B b // NB: if "b: B {}", it should be a recursion error
    }
    component B : Item {
      property A a
    }
    A {
      b: B { Component.onCompleted: console.log(a) }
    }
  }
  

• could be troublesome in the code resolving names to types (e.g. is Foo.Bar an enum.enumerator or an inline component?)