unchecked casts and intersection types

[Liam Miller-Cushon]

I was surprised by the unchecked warning in the following code. Is this
behaviour correct?

It stems from Types.isCastable using the same logic for union and
intersection types, and in both cases requiring that each element type be
castable to the target. For intersection types, isn't it sufficient for any
element type to be castable?

class Test {
  interface A<Y> {}
  interface B<Z> extends A<Z> {}
  interface C {}

  <T extends A<String> & C> B<String> f(T t) {
    B<String> result;

    A<String> a = t;
    result = (B<String>) a; // ok

    result = (B<String>) t; // unchecked
    // required: B<String>
    // found:    T
    // where T is a type-variable:
    //   T extends A<String>,C declared in method <T>f(T)

    return result;
  }
}
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