Towards Improved GADT Reasoning in Scala

Generalized algebraic data types (GADT) have been notoriously difficult to implement correctly in Scala. Both major Scala compilers, Scalac and Dotty, are currently known to have type soundness holes related to them. In particular, covariant GADTs have exposed paradoxes due to Scala's inheritance model. We informally explore foundations for GADTs within Scala's core type system, to guide a principled understanding and implementation of GADTs in Scala.