Using type alias causes error where directly using the type doesn't

[jfharden]

It is surprising (at least to me) that in the following pkl definition:

typealias Step = GetStep | PutStep

class Job {
  steps: Listing<Step>?
}

class GetStep {
  prop1: Step?
}

class PutStep {
  prop1: Step?
}

An error is thrown for prop1 on the PutStep saying:

–– Pkl Error ––
Type alias definitions must not be cyclic.

8 | prop1: Step?
           ^^^^
at foo#GetStep (file:///Users/jonathan.harden/test/pkl/foo.pkl, line 8)

1 | typealias Step = GetStep | PutStep
                     ^^^^^^^
at foo#Step (file:///Users/jonathan.harden/test/pkl/foo.pkl, line 1)

4 | steps: Listing<Step>?
                   ^^^^
at foo#Job (file:///Users/jonathan.harden/test/pkl/foo.pkl, line 4)

106 | text = renderer.renderDocument(value)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
at pkl.base#Module.output.text (https://github.com/apple/pkl/blob/0.25.2/stdlib/base.pkl#L106)

However if instead of the typealias, I use the value of the typealias directly it is fine:

class Job {
  steps: Listing<GetStep | PutStep>?
}

class GetStep {
  prop1: (GetStep | PutStep)?
}

class PutStep {
  prop1: (GetStep | PutStep)?
}

It seems I should be able to use a typealias anywhere I would use a type and get the same result.

I can't see in the documentation for type alias where this is mentioned.

Clearly the workaround is to not use a type alias and instead just redefine the complex type everywhere which is ok for now, but it's a surprising limitation.

Versions:

$ pkl --version
Pkl 0.25.2 (macOS 14.1, native)

[stackoverflow]

We are discussing the possibility to create recursive type aliases. But, indeed, this is not currently supported.