[GROOVY-10250] Erroneous type inference of closure in STC

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.0.9
• Fix Version/s: 4.0.0-beta-2
• Component/s: Static Type Checker
• Assignee: Eric Milles
• Reporter: Christopher Smith
• Created: 21/Sep/21 20:23
• Updated: 03/Feb/22 22:32
• Resolved: 26/Sep/21 22:19

Description

Sorry if this is a duplicate, but I couldn't identify it. I'm using the Vavr Option and Either types here:

repository.findOne(id) // <- Option<Foo>
  .toEither(config.&defaultUri) // <- Either<URI, Foo>
  .map(linkProvider.&forFoo) // <- Either<URI, URI>
  ...

This is the signature of the method being called:

<L> Either<L, T> toEither(Supplier<? extends L> leftSupplier)

At the invocation of toEither, the STC appears to correctly recognize that config.&defaultUri is a valid match for Supplier<URI> and approves the call. However, instead of inferring L=URI it infers L=Closure<URI>, apparently failing to "dereference" the closure into the functional interface, so that later calls expecting <URI, URI> fail because the STC thinks that Left is a closure.

Inserting a type witness at toEither does seem to work, which I think is new with 3.0.9.

*Update:* Optimism apparently premature. The problem appears to be that in fact toEither is overloaded, and the static compiler is selecting the immediate-value override instead of the Supplier override.

Bizarrely (I would open a second ticket on this but have no idea how to cleanly repro), when I changed .& to :: just to see what would happen, I get a ClassFormatError under GRECLIPSE 4.3.0.v202108132133-e2106 (attached) and a stack overflow under groovyc 3.0.9:

    at org.codehaus.groovy.transform.stc.StaticTypeCheckingVisitor.inferMethodReferenceType (StaticTypeCheckingVisitor.java:3615)    at org.codehaus.groovy.transform.stc.StaticTypeCheckingVisitor.visitMethodCallArguments (StaticTypeCheckingVisitor.java:2750)    at org.codehaus.groovy.transform.stc.StaticTypeCheckingVisitor.inferMethodReferenceType (StaticTypeCheckingVisitor.java:3646)    at org.codehaus.groovy.transform.stc.StaticTypeCheckingVisitor.visitMethodCallArguments (StaticTypeCheckingVisitor.java:2750)    at org.codehaus.groovy.transform.stc.StaticTypeCheckingVisitor.inferMethodReferenceType (StaticTypeCheckingVisitor.java:3646)

Explicitly providing this form does work, and appears to be a substantial improvement in 3.0.9's generics handling (is this the partial backport you meant?), because now the entire generic chain infers properly. I note that previously, the STC ignored generics in the as clause and always treated types as raw, but now it observes them, and I didn't gather this from the release notes.