FULL PRODUCT VERSION :
java version "9.0.1"
Java(TM) SE Runtime Environment (build 9.0.1+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.1+11, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Linux fede-peralta 4.13.0-25-generic #29-Ubuntu SMP Mon Jan 8 21:14:41 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
Consider the following code:

public class Ambiguous3 {
    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 Ambiguous3 and method ok(Runnable) in Ambiguous3 match”.

But just changing the order of the methods:

public class Ambiguous3 {
    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.

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.

See the issue in Stackoverflow here:
https://stackoverflow.com/a/48260046/1876620


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Just try to compile the first version of the class.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Code should compile irrespective of methods' declaration order.
ACTUAL -
Code doesn't compile in some cases, depending on the order of declaration of methods.

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
Arrange the methods in a different order within the class, use casts to hint the compiler about which method should be chosen, turn lambda expression to statement instead of expression and the like...