reference to method is ambiguous when migrating from java8 to java9

Question

I'm migrating a project from JAVA 8 to JAVA 9 and I'm having some trouble getting the code to work. All work in JAVA 8 but in 9 I'm having the following errors:

Error java: reference to ok is ambiguous
      both method <T>ok(java.util.function.Supplier<T>)  and method ok(web.Procedure) match

here is the code when I'm calling the method:

public ResponseEntity<List<MailTemplateDto>> mailTemplateFindAll() {
    return ok(() -> mailTemplateService.findAll());
}

and here is the implementation :

 public <T> ResponseEntity<T> ok(Supplier<T> action) {
     return this.body(HttpStatus.OK, action);
 }

 public <T> ResponseEntity<T> ok(T body) {
     return this.ok(() -> {
         return body;
     });
 }

 public ResponseEntity<Void> ok(Procedure action) {
     action.invoke();
     return this.status(HttpStatus.OK);
 }

 public ResponseEntity<Void> ok() {
     return this.status(HttpStatus.OK);
 }

code for Procedure interface:

@FunctionalInterface
public interface Procedure {
    void invoke();
}

Any ideas?

---

Reproducible Code ::

public class Q48227496 {

    public A<?> test() {
        return ok(() -> System.out.append("aaa"));
    }

    private class A<T> {
    }

    private <T> A<T> ok(java.util.function.Supplier<T> action) {
        return new A<>();
    }

    public <T> A<T> ok(T body) {
        return new A<>();
    }

    private <T> A<T> ok(Procedure action) {
        return new A<>();
    }

    public <T> A<T> ok() {
        return new A<>();
    }

    @FunctionalInterface
    public interface Procedure {
        void invoke();
    }
}

Resulting in the following error with java9 compiler::

error: reference to ok is ambiguous
        return ok(() -> System.out.append("aaa"));
               ^
  both method <T#1>ok(Supplier<T#1>) in Q48227496 and method <T#2>ok(Procedure) in Q48227496 match
  where T#1,T#2 are type-variables:
    T#1 extends Object declared in method <T#1>ok(Supplier<T#1>)
    T#2 extends Object declared in method <T#2>ok(Procedure)

Answer 1

This is a bug.

It has been reported with bug ID : JDK-8195598

---

I simplified your example further:

public class Q48227496 {
    public CompletableFuture<?> test() {
        return ok(() -> System.out.append("aaa"));
    }
    public <T> CompletableFuture<T> ok(Supplier<T> action) {
        return CompletableFuture.supplyAsync(action);
    }
    public <T> CompletableFuture<T> ok(T body) {
        return CompletableFuture.completedFuture(body);
    }
    public CompletableFuture<Void> ok(Runnable action) {
        return CompletableFuture.runAsync(action);
    }
}

This fails in the release version of Java 9 with “reference to ok is ambiguous”, stating “both method <T>ok(Supplier<T>) in Q48227496 and method ok(Runnable) in Q48227496 match”.

But just changing the order of the methods

public class Q48227496 {
    public CompletableFuture<?> test() {
        return ok(() -> System.out.append("aaa"));
    }
    public <T> CompletableFuture<T> ok(T body) {
        return CompletableFuture.completedFuture(body);
    }
    public <T> CompletableFuture<T> ok(Supplier<T> action) {
        return CompletableFuture.supplyAsync(action);
    }
    public CompletableFuture<Void> ok(Runnable action) {
        return CompletableFuture.runAsync(action);
    }
}

causes the compiler to accept the code without any errors.

So, obviously, this is a compiler bug as the order of the method declarations should never have an impact on the validity of the code.

Also, removing the ok(T) method makes the code accepted.

Note that whenever the compiler accepts the code, it considers ok(Supplier) to be more specific than ok(Runnable), which is the expected behavior for a function parameter that matches both.