A TypeTest[S, T] can serve as an extractor that matches if and only if a value of type S is an instance of T.
A TypeTest[S, T] can serve as an extractor that matches if and only if a value of type S is an instance of T.
The compiler tries to turn unchecked type tests in pattern matches into checked ones by wrapping a (_: T) type pattern as tt(_: T), where tt is the TypeTest[S, T] instance. Type tests necessary before calling other extractors are treated similarly. SomeExtractor(...) is turned into tt(SomeExtractor(...)) if T in SomeExtractor.unapply(x: T) is uncheckable, but we have an instance of TypeTest[S, T].