That strikes me as a good example of where the kinds of features JetBrains is known for don't really mesh with Objective-C.

If doSomethingElse is a method defined in a category I expect to be loaded at runtime, or something I expect to be caught by foo's forwardInvocation method, having it turn red and bug me to provide an implementation in Foo is exactly the wrong thing to do.

It's the kind of little feature that really only makes sense in the context of a static language like C#, where a method call without a corresponding method implementation can be known definitively to be an error.