[GROOVY-9762] Wrong type resolved if method reference used with typed method

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 3.0.5
• Fix Version/s: 3.0.7, 4.0.0-alpha-2, 2.5.19
• Component/s: Compiler
• Assignee: Eric Milles
• Reporter: Marcin Zajaczkowski
• Created: 28/Sep/20 20:44
• Updated: 06/Sep/22 15:08
• Resolved: 09/Oct/20 05:19

Description

I've encountered a problem with type checking if a method reference is used.

In following (stripped down from the original production case) code in the line with the error variable the following error is reported:

> Task :compileTestGroovy
startup failed:
/home/foobar.../MethodReferenceProblemWithType.groovy: 15: [Static type checking] - Cannot assign value of type java.lang.Object to variable of type java.util.List <Integer>
 @ line 15, column 31.
 List<Integer> error = opt.map(this::just).get()  

@CompileStatic
class MethodReferenceProblemWithType {

    static void reproduce() {
        Optional<Integer> opt = Optional.ofNullable(Integer.valueOf(1))
        List<Integer> ok = opt.map(o -> just(o)).get()    //ok
        List<Integer> error = opt.map(this::just).get()   //problem
    }
    private static <T> List<T> just(T data) {
        return new ArrayList<T>();
    }
}  

If `@CompileStatic` is removed, I still see a type incompatibility warning in Idea. Switching to lambda is a workaround, therefore it's just a minor glitch. Tested with Groovy 3.0.5.

Issue Links

• is related to: GROOVY-7992 Sort methods that accept a comparator should accept Comparator<? super T> (fix type checker ability to cope with super in DGM methods)