Unnamed union with named members on the model level

[pawel-kow]

Clear and concise description of the problem

Problem as stated in #7124

Scenario: model foo has a property bar and one of 2 properties baz and fox.

Potential modelling (or any equivalent that would allow anonymous reference to baz_or_fox where the union properties are named but are directly placed in foo):

model foo {
  bar: uint32;
  ...baz_or_fox;
}

union baz_or_fox {
  baz: uint32;
  fox: uint32;
} 

Currently it throws "Cannot spread properties of non-model type.".

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