`parent` key is not an actually is expression

[Mingun]

Analysing code when working on my own implementation of the compiler, I found that the parent key is extracted as an expression, but actually the only possible expressions are:

• false
• _parent
• _root
• _root._parent
• _root._parent._parent
• ...

That expressions although by the form looks acceptable, is rejected by the compiler:

• param where

  params:
    - id: param
      type: struct
  

  Error:

  io.kaitai.struct.precompile.TypeMismatchError: parent=Name(identifier(param)) is expected to be either of user type or false, but CalcKaitaiStructType found

• true ? _root : _parent Error:

  io.kaitai.struct.precompile.TypeMismatchError: parent=IfExp(Bool(true),Name(identifier(_root)),Name(identifier(_parent))) is expected to be either of user type or false, but CalcKaitaiStructType found

• [_root, _parent][1] Error:

  io.kaitai.struct.precompile.TypeMismatchError: parent=Subscript(List(WrappedArray(Name(identifier(_root)), Name(identifier(_parent)))),IntNum(1)) is expected to be either of user type or false, but CalcKaitaiStructType found

This is happened because:

• parent should have type of the UserType
• the type of all that expressions is inferred as CalcKaitaiStructType
• the UserType extends StructType and the CalcKaitaiStructType extends StructType, so they does not have the common ancestor

If parent should be a true expression, than this looks like a bug. If parent should only allow the currently allowed usage variants, it is better to rewrite it's parsing to be more straightforward.

What the desired behaviour?