[GROOVY-6479] Closure to SAM interface coercion doesn't handle contravariant types correctly

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 2.3.0-beta-1
• Component/s: Static Type Checker
• Assignee: Cédric Champeau
• Reporter: Luke Daley
• Created: 11/Dec/13 03:42
• Updated: 05/Apr/14 00:00
• Resolved: 15/Jan/14 11:15

Description

When coercing a closure to an interface where the method takes a contravariant type, Groovy should create an implementation that accepts the most special type.

class TestCase {

  interface Action<T> {
    void execute(T thing)
  }

  static class Wrapper<T> {

    private final T thing

    Wrapper(T thing) {
      this.thing = thing
    }

    void contravariantTake(Action<? super T> action) {
      action.execute(thing)
    }

    void invariantTake(Action<T> action) {
      action.execute(thing)
    }

  }

  static <T> Wrapper<T> wrap(Callable<T> callable) {
    new Wrapper(callable.call())
  }

  static Integer dub(Integer integer) {
    integer * 2
  }

  static void main(String[] args) {
    wrap {
      1
    } contravariantTake {
      dub(it) // fails static compile, 'it' is not known to be Integer
    }

    wrap {
      1
    } invariantTake {
      dub(it) // passes static compile, 'it' is known to be Integer
    }
  }

}

Issue Links

• depends upon: GROOVY-5924 Provide a mechanism to allow the type checker to know what are the expected argument types of a closure