It is often useful to distinguish between synchronous and asynchronous exceptions. The common idiom is to run a user-supplied computation catching any synchronous exceptions but allowing asynchronous exceptions (such as user interrupt) pass through. There's no way to know how — synchronously or asynchronously — an exception was thrown, so we have to work around it by relying on the exception type itself. Unfortunately, the AsyncException type was designed as a leaf in the exceptions hierarchy, so it's not possible to extend it with user-defined asynchronous exception types. Thus I propose to add SomeAsyncException as an extensible type ("class") of asynchronous exceptions. It will be a direct subclass of SomeException, and will become the superclass of the existing AsyncException type. Users can then define their own asynchronous exceptions by subclassing SomeAsyncException. My main motivation for this change is exceptions used for timeouts. So the second part of this proposal it to mark System.Timeout.Timeout as an asynchronous exception. Roman