Safe cast is rejected (with warning) by javac

FULL PRODUCT VERSION :
java version "1.6.0"
Java(TM) SE Runtime Environment (build 1.6.0-b105)
Java HotSpot(TM) Client VM (build 1.6.0-b105, mixed mode, sharing)

A DESCRIPTION OF THE PROBLEM :
Javac rejects a cast that is perfectly safe even without a warning.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile attached program and observe that it fails

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Should compile without warning.
ACTUAL -
Compiler rejects program

ERROR MESSAGES/STACK TRACES THAT OCCUR :
cast.java:4: incompatible types
found : java.lang.Iterable<capture of ? extends java.lang.Number>
required: java.util.Collection<? extends java.lang.Number>
  Collection<? extends Number> x2 = x1; // javac reports error here
                                    ^
REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.util.*;
class cast {
    Iterable<? extends Number> x1 = null;
    Collection<? extends Number> x2 = x1; // javac reports error here
}

---------- END SOURCE ----------
Actually the problem is the following (I checked with Neal Gafter, the submitter of this bug)

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

ACTUAL BEHAVIOR

The output is (both 6-open and 6u06)

$ javac Test.java -Xlint
Test.java:4: warning: [unchecked] unchecked cast
found : java.lang.Iterable<capture#156 of ? extends java.lang.Number>
required: java.util.Collection<? extends java.lang.Number>
   Collection<? extends Number> x2 = (Collection<? extends Number>)x1; //javac generates a warning here
                                                                   ^
1 warning

EXPECTED BEHAVIOR

The program should compile without any warning.