Consider two upper bounds, at least one of which mentions an inference variable:
a <: Foo<b>
a <: Bar<String>

If these upper bounds share a parameterized type, we can infer additional constraints from the type arguments (e.g., maybe b = String).

Here's an example (notably, this compiles in 7, due to a quirk in javac's implementation):

interface Task<E extends Exception> {}
class Comparator<T> {}
class CustomException extends Exception {}

class TaskQueue<E extends Exception, T extends Task<E>> {}

abstract class Test {
 abstract <E extends Exception, T extends Task<E>>
     TaskQueue<E, T> create(Comparator<? super T> comparator);

 void f(Comparator<Task<CustomException>> comp) {
   TaskQueue<CustomException, Task<CustomException>> queue = create(comp);
 }
}

Reported on lambda-dev: http://mail.openjdk.java.net/pipermail/lambda-dev/2013-November/011442.html