[GROOVY-10844] STC mis-tracks generics in type hierarchy

• Type: Bug
• Status: Closed
• Priority: Critical
• Resolution: Fixed
• Affects Version/s: 4.0.6
• Fix Version/s: 4.0.7
• Component/s: Static compilation, Static Type Checker
• Assignee: Eric Milles
• Reporter: Christopher Smith
• Created: 16/Nov/22 16:55
• Updated: 10/Feb/23 21:40
• Resolved: 27/Dec/22 15:49

Description

I have a Vavr Try pipeline of this form:

class AttachmentSpec {
  // fields
  Attachment toAttachment(Ref ref) { ... }
}


Try.success(initialValue)
  .mapTry { process(it) } // returns a Map
  .map { convertToRef(it) } // returns a Ref.File extends Ref
  .recover { new Ref.Uri(it.href) } // Ref.Uri extends Ref
  .map(spec::toAttachment)

On a failure in process, this pipeline should enter recover and supply a Ref.Uri, then do the final map. However, this fails with a ClassCastException: cannot convert Ref.Uri to Ref.File. Apparently, the compiler infers Ref.Uri as the return value from the first map (which is legitimate), but then it fails to produce a compile-time error when recover presents a then-incompatible Ref.File. The compiler should either (ideally) recognize that the desired type is superclass Ref or, more simply but at least correctly, produce a compile-time error until a type witness is inserted (and observed).