What types are special to the Scala compiler or runtime?

From the URL:

Q: Scala makes a big deal about how what seem to be language features are implemented as library features. Is there a list of types that are treated specially by the language?

A: The following types are crucial to Scala's type system. They have an influence on how type checking itself is performed.

It’s interesting that you can do some research on this by looking at Definitions.scala.