It would be useful to have an Xlint warning when a type-variable's declared bound(s) is final as in:

class Foo<X extends Integer> {
 X x;
}

I think this is a useful one, as it helps the programmer get rid of useless generic types (in this case you only have to replace X with Integer inside Foo). The above code could be simplified to:

class Foo {
 Integer x;
}