Ambiguity.java

import java.util.Collection;

public class Ambiguity {
	public void foo(CriteriaBuilder builder) {
    // change Object -> Number (or any other type) and it compiles
		Expression<Object> objectExpression = null;
		Expression<Collection<Object>> collectionOfObjectExpression = null;
		builder.isNotMember(objectExpression, collectionOfObjectExpression);
	}
}

class Expression<T> { }

class CriteriaBuilder {
  public <E,C extends Collection<E>> void isNotMember(E elem, Expression<C> collection) {}
  public <E,C extends Collection<E>> void isNotMember(Expression<E> elem, Expression<C> collection) {}
}

It still doesn't entirely make sense to me:

1. To consider the first method a match, E has to be bound to Expression<Object>. That'd mean the second parameter is expected to be an Expression<Collection<Expression<Object>>>. But we hand in a Expression<Collection<Object>> so that shouldn't match, should it?
2. There is no Java 6 involved at all. It also compiles if you use a JDK 8 compiler but set the source and target level to 1.6.
3. My actual question is: if I have the two variables of type Expression<Object> and Expression<Collection<Object>>, how do I tell the compiler I want to bind to the second method? There doesn't seem to be any way (through type hints, casts or the like) to get this to work.

@otrosien has a workaround in the other gist I created: https://gist.github.com/olivergierke/c644327bb89191f2b52e298427ebf201