Is your feature request related to a problem?

The main use-case for union types is to describe the type of parameters of external declarations. For now, the effort to allow working with values that have a union type inside the DSL does not seem worth it.

Desired solution

Allow them only as the type of

• parameters of annotations,
• parameters of classes,
• parameters of functions.

Since parameters of enum variants can be accessed as members, we must omit them.

Possible alternatives (optional)

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Additional Context (optional)

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