In 2.11 there seems to be an inheritance problem with it:
```
[error] /Users/georgii/workspace/util/util-core/src/main/scala/com/twitter/util/Promise.scala:36: ambiguous reference to overloaded definition,
[error] both value parent in class DetachablePromise of type com.twitter.util.Promise[_ <: A]
[error] and  method parent in class Promise of type => com.twitter.util.Promise[A]
[error] match expected type ?
[error]     parent.continue(this)
[error]     ^
[error] /Users/georgii/workspace/util/util-core/src/main/scala/com/twitter/util/Promise.scala:38: ambiguous reference to overloaded definition,
[error] both value parent in class DetachablePromise of type com.twitter.util.Promise[_ <: A]
[error] and  method parent in class Promise of type => com.twitter.util.Promise[A]
[error] match expected type ?
[error]     def detach(): Boolean = parent.detach(this)
[error]                             ^
[error] two errors found
[error] (util-core/compile:compile) Compilation failed
[error] Total time: 15 s, completed Apr 30, 2014 11:13:53 AM
```