I'm probably being a bit too pedantic here, but strictly speaking they don't need to be part of the language at all. You just need basic support for (generic/parametric) algebraic data types.

For example, Haskell has Maybe which is not part of the language nor treated specially by the compiler in any way. It is part of the Prelude (think "standard library"), so that (basically) all libraries can agree on what "Maybe a" means.