This is a bug in the jsr14 prototype compiler.

frog$ cat -n T.java
     1 interface Collection<E> {
     2 <T> T[] toArray(T[] a);
     3 }
     4
     5 interface Set<E> extends Collection<E> {
     6 <T> T[] toArray(T[] a);
     7 }
     8
     9 interface SortedSet<E> extends Set<E> {
    10 }
    11
    12 class TreeSet<E> {
    13 public void addAll(Collection<E> c) {
    14 if (c instanceof SortedSet<E>) {}
    15 }
    16 }
frog$ make
/java/jdk/ws/tools/jsr14/build/solaris-sparc/bin/javac -gj *.java
T.java:14: interfaces Collection<E> and SortedSet<E> are incompatible; both define toArray(T[]), but with different return type
        if (c instanceof SortedSet<E>) {}
            ^
1 error
*** Error code 1
make: Fatal error: Command failed for target `all'
frog$