Code that compiles in java 7 does not compile in Java 8. Please provide an explanation

Question

Why does the following code do not compile in Java 8. I know type inference is the culprit here, But I would like to have an explanation.

public class TypeInferenceProblem {

    class ATest<E extends B>
    {

        private E find(C<? extends E> CObj)
        {
            return null;
        }

        public void findCs(List<? extends C<? extends E>> cList)
        {

            find(new C());// This compiles fine
            for (C cObj : cList)
                {
                    E cachedEntity = find(cObj); // This cause error in java 8 but works fine in java 7
                }
        }
    }

     class B{
     }

     class C <T> {
     }
}

Answer 1

Just to give definite references to Holger's correct comments:

In JLS 8 this is determinged in §18.5.2, which contains this sentence (inside the 6th major bullet):

If unchecked conversion was necessary for the method to be applicable during constraint set reduction in §18.5.1, then the parameter types of the invocation type of m are obtained by applying θ' to the parameter types of m's type, and the return type and thrown types of the invocation type of m are given by the erasure of the return type and thrown types of m's type.

I highlighted the relevant piece.

A similar sentence was already present in JLS 7 §15.12.2.6:

Otherwise, if unchecked conversion was necessary for the method to be applicable, then the result type is the erasure (§4.6) of the method's declared return type.

Both versions amount to defining that an invocation of find(..) with a raw type argument (which requires unchecked conversion) has a return type that is obtained by erasing the declared return type E to B.

If compilers for Java 7 did not report this error, then this was a bug.