Problem when casting from parametrized type to concrete class

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)

ADDITIONAL OS VERSION INFORMATION :
Windows XP Proffesional

A DESCRIPTION OF THE PROBLEM :
Can not compile class containing casting from the parametrized type to a concrete type.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile following class:

--------------------------------------------------------------

import java.util.Comparator;

public class GenericsComparator <T extends Comparable<T>> implements Comparator<T>{
public int compare(T o1, T o2) {
if (o1 instanceof String)
{
String s = (String)o1;
}
return 0;
}
}

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Class should be compilable.
ACTUAL -
>javac deleteme\GenericsComparator.java

deleteme\GenericsComparator.java:8: inconvertible types
found : T
required: java.lang.String
                if (o1 instanceof String)
                    ^
deleteme\GenericsComparator.java:10: inconvertible types
found : T
required: java.lang.String
                        String s = (String)o1;
                                           ^
2 errors

ERROR MESSAGES/STACK TRACES THAT OCCUR :
inconvertible types

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.util.Comparator;

public class GenericsComparator <T extends Comparable<T>> implements Comparator<T>{
public int compare(T o1, T o2) {
if (o1 instanceof String)
{
String s = (String)o1;
}
return 0;
}
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Explicit cast to Object:

------------------------------------------------

public class GenericsComparator <T extends Comparable<T>> implements Comparator<T>{

public int compare(T o1, T o2) {
if ((Object)o1 instanceof String)
{
String s = (String)(Object)o1;
}
return 0;
}
}