[Merged the description from 6717221]

Consider the following program:

import java.util.*;
class cast {
  Iterable<? extends Number> x1 = null;
  Collection<? extends Number> x2 = (Collection<? extends Number>)x1; // javac generates a warning here
}

The JLS seems to mandate a warning (JLS 5.5) for a cast that can be considered 'statically-safe':

"A cast from a type S to a parameterized type (§4.5) T is unchecked unless at least one of the following conditions hold:
1) S <: T.
2) All of the type arguments (§4.5.1) of T are unbounded wildcards.
3) T <: S and S has no subtype XT , such that the erasures (§4.6) of X and T are the same. "

Here we have that:

1) S (Iterable<? extends Number>) is *not* a subtype of T (Collection<? extends Number>)
2) T's type argument (? extends Number) is *not* an unbounded wildcard
3) There are *infinite* subtypes of S of the kind J = Collection<? extends K>, with K <: Number, such that |J| == |T|

Thus the JLS wants the cast to raise a warning, when there is no need. In the following program, the JLS also wants a warning and javac doesn't give one:

class I1<T> {}
class I2<T> extends I1<T> {}
class Main {
  void f(I1<Integer> i1) { Object o = (I2<? extends Number>) i1; }
}

The code is safe and no warning is logically "necessary" because no heap pollution can result from the cast at runtime, but it is a shortcoming of the specification that it doesn't justify javac's behavior. In short, it is both a javac bug (it doesn't comply with the specification) and a specification bug (the spec should justify javac's current behavior).

This area of the JLS would benefit from an overhaul.

On 1/7/07, ... wrote:
We issue an unchecked cast warning on this:
        
        interface I1<T> {}
        class GC22<T> implements I1<T> {}
        class A {
                 I1<Short>[] x = null;
                 GC22<? extends Number>[] y = (GC22<? extends Number>[])x;
        }
        
        but javac doesn't. Is this because the rules in JLS 5.5 about unchecked
        casts refer to parameterized types, and an array of a parameterized type
        doesn't count as a parameterized type?