Dart does this, or at least something very similar, and it is really nice. It's most useful for narrowing nullable types to their non-nullable counterparts after a null-check, but the subclass case works too.
As your sibling allready stated, in Kotlin it is known as smart casting. Good to see that the article allready recognizes the prior art from ML which was conceived in 1971.
Dart’s “..” operator, which allows void-returned function calls to be chained, adds no performance cost to the code and is one of my favorite pieces of Dart.
