Add validation for mixing fixed and additionalProps

[bhelx]

This is a place where OpenAPI and XTP diverge so let's make an explicit error message

[nilslice]

LGTM!

Just curious, if someone did want to do roughly this, they would instead add another fixed property that is actually a $ref to a map ?

[bhelx]

There isn't really a way to model this right now. I'm not sure how we'd model it in every typed languages. In typescript it's simple. in rust maybe, in go, csharp, c no idea? But we will look into it. The recommendation i'd give would be to move the untyped parts into their own property. Example:

   MixedObject:
      description: should not allow mixing fixed and additional props for now
      properties:
        hello:
          type: string
      additionalProperties:
        type: string

becomes

   MixedObject:
      description: should not allow mixing fixed and additional props for now
      properties:
        hello:
          type: string
        metadata:
          type: object
          additionalProperties:
            type: string

but of course if you can't control what the shape looks like, might be tricky.

[nilslice]

Ah that's even better than I had assumed! Thanks