A DESCRIPTION OF THE REQUEST :
There are many cases where an object reference gets inadvertently assigned to itself (see example).  There is no reason whatsoever for the programmer doing this, since it is an ineffectual operation.  Therefore, the operation almost always causes a bug.  In IDE's such as Eclipse, this produces a complie warning.  Based on the scope of the variable, the compiler could eliminate this problem entirely by recognizing this as a compile error.

JUSTIFICATION :
I have seen an unusually large number of bugs of this type produced by developers who, in general, ignore compile errors that may or may have not been produced by their particular IDE.  While there are workarounds such as alternative naming conventions, reconfiguring an IDE to mark this as an error, etc., making this an error would absolutely prevent programmers from making this common mistake, thereby reducing bugs in general.  I have never seen an occurrence of self-assigning an object reference that was intentional and can't imagine any reason for doing so.  Yes, it would break existing code, but only a) bugs or b) ineffectual lines of code.  Just flip on this option in Eclipse to regard this as an error in any very large Java project and you will likely find several bugs.  IMHO, this should always be a compile error produced by the JDK.


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
A compile error stating that the assignment has no effect.
ACTUAL -
Nothing, although some IDE's such as Eclipse produce a compile warning.

---------- BEGIN SOURCE ----------
// Example.java
public class Example {
    private int foo;
    public void setFoo(int foo) {
        foo = foo; // assignment has no effect due to improper scope
    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Alternative naming conventions, or reconfiguring an IDE to mark this as an error.  However, these are proactive solutions that developers in general don't follow.