Type should not be able to extend choice type

[SimonCockx]

Due to the implementation strategy of choice types as a thin layer of syntactic sugar on top of complex types, a bug was introduced, allowing types to extend choice types, e.g.,

choice StringOrNumber:
  string
  number

type StringOrNumberWithAttribute extends StringOrNumber:
  attr int (1..1)

We should deprecate this behaviour and fix it.

Impact on the CDM

Unfortunately, this bug is already used as a feature in the CDM:

type BasketConstituent extends Observable:
  ...

type TransferableProduct extends Asset:
  ...

I would propose to refactor them as follows:

type BasketConstituent:
  observable Observable (1..1)
  ...

type TransferableProduct:
  asset Asset (1..1)
  ...

If BasketConstituent is considered a kind of Observable, it should be added to the list of options inside Observable:

choice Observable:
  ...
  BasketConstituent

Similarly, if TransferableProduct is considered a kind of Asset, it should be added to the list of options inside Asset:

choice Asset:
  ...
  TransferableProduct