Compiler ambiguity when compiling with target JDK 8

AmbiguitySample.java

public class AmbiguitySample {

  /**
   * This compiles when using a JDK 8 with target JDK 6 but fails when the target is JDK 8.
   */
	public static void main(String[] args) {

		CriteriaBuilder builder;
		Expression<Object> objectExpression;
		Expression<Collection<Object>> collectionOfObjectExpression;

		// Compiler error: AmbiguitySample.java:[35,24] reference to isNotMember is ambiguous
		// [ERROR] both method
		// <E,C>isNotMember(javax.persistence.criteria.Expression<E>,javax.persistence.criteria.Expression<C>) in
		// javax.persistence.criteria.CriteriaBuilder and method
		// <E,C>isNotMember(E,javax.persistence.criteria.Expression<C>) in javax.persistence.criteria.CriteriaBuilder match
		builder.isNotMember(objectExpression, collectionOfObjectExpression);
		//      ^^^^^^^^^^^
	}
}

This problem kept me interested a while... It seems like a problem in "target-type inference" from java8. See: http://stackoverflow.com/questions/32294140/java-8-reference-to-method-is-ambiguous . Java8 is using target-type inference to infer the type of E, but there must be an exception when using <Object> as generic type, because if you use something different/more specific, it will work. Here is another working alternative:

    void test(CriteriaBuilder builder, 
            Expression<? extends Object> objectExpression, 
            Expression<? extends Collection<Object>> collectionOfObjectExpression) {
        builder.isNotMember(objectExpression, collectionOfObjectExpression);
    }

You can also trick the compiler into the correct behavior with this piece of code:

    <T> void test(CriteriaBuilder builder, Expression<T> objectExpression,
    Expression<Collection<T>> collectionOfObjectExpression) {
        builder.isNotMember(objectExpression, collectionOfObjectExpression);
    }

Sorry, I've been in a hurry and thought I'd just oversee something. I've no prepared a reproducing example here. Run mvn clean compile see it fail. Change the java.version property to 1.6 see it compile.

$ java -version
java version "1.8.0_112"
Java(TM) SE Runtime Environment (build 1.8.0_112-b16)
Java HotSpot(TM) 64-Bit Server VM (build 25.112-b16, mixed mode)

@otrosien — once again, you made my day. Thanks for that!