Back in 2020, I created Cubiml, a simple ML-like language that demonstrated how to extend the usual Hindley–Milner type system with subtyping while still having decidable full type inference. One question I got was whether it would be possible to support generalized algebraic data types (GADTs) in Cubiml. I had heard that GADTs break type inference and didn’t see the point, so I didn’t think much of it at the time.
