* Casting contexts allow the use of ***one of the following***:
- a widening and narrowing primitive conversion (5.1.4)
    // Missing; reported by Neal and J.Stephen Adamcyzk
- a boxing conversion (5.1.7) optionally followed by a widening reference conversion (5.1.5)
    // Extension of existing clause
- an unboxing conversion (5.1.8) optionally followed by a widening primitive conversion (5.1.2)
    // Extension of existing clause
- a widening reference conversion (5.1.5) optionally followed by either an unboxing conversion (5.1.8) or an unchecked conversion (5.1.9)
    // Extension of existing clause, as per 6558543
- a narrowing reference conversion (5.1.6) optionally followed by either an unboxing conversion (5.1.8) or an unchecked conversion (5.1.9)
    // Extension of existing clause, as per 6558543

(The detailed rules for casting must also change to reflect the updated list.)


* A cast from a type S to a type variable T is unchecked unless S<:T
(Not "A cast to a type variable is always unchecked.")


* If S is an interface type:
    If T is an array type, then a compile-time error occurs unless S is the type java.io.Serializable or the type Cloneable, the only interfaces implemented by arrays.
    If T is a type that is final, then:
      If S is not a parameterized type or a raw type, then T must implement S, or a compile-time error occurs.


When the source type is an intersection type S = A1 & A2 & ... & An, the success of the cast should be determined by the most restrictive component of the intersection type. That is, if there exists an Ai (i:1..n) which is not convertible into the target type T, then the cast should fail. (Corresponds to compiler CR 6557182; see comments therein.)

Josh reported a problem where highlights the lack of cast conversion rules for nested types:

class Outer<E> {
    Inner inner;
    Outer(E e) { inner = new Inner(e); }

    class Inner {
        E e;
        Inner(E e) { this.e = e; }
        E getOtherElement(Object other) {
            Inner that = (Inner) other; // Shouldn't this generate a warning?
            return that.e;
        }
    }
    public static void main(String[] args) {
        Outer<String> s = new Outer<String>("hovercraft");
        Outer<Integer> i = new Outer<Integer>(1234);
        // The next line produces a ClastCastException at runtime!
        String producesClassCast = s.inner.getOtherElement(i.inner);
    }
}

It compiles, as of javac 1.6.0_06, without generating a warning, and the compiler-generated cast fails at runtime for:
 
    String producesClassCast = s.inner.getOtherElement(i.inner);
 
This appears to violate Java's "cast iron guarantee." Luckily, the compiler is simply failing to generate an unchecked cast warning on line 16:
 
    Inner that = (Inner) other; // Shouldn't this generate a warning?

The problem is that Inner (in this context) refers to Outer<E>.Inner, so we're casting from Object to a parameterized type. JLS clarification is required. javac in JDK7 rightly gives a warning:

Test.java:12: warning: [unchecked] unchecked cast
found : java.lang.Object
required: Outer<E>.Inner
           Inner that = (Inner) other; // Shouldn't this generate awarning?
                                ^