@ponylang/committer see: https://pony.groups.io/g/user/message/1077

I had a look into this, with a slightly reduced test case :

class A
interface State[X, Y]
   fun val bind(next: State[Y, A ref])

The unconstrained type params are actually constraint by themselves, so the constraints are X: X, Y: Y.

The loop in reify reifies the constraint by replacing each type variable by its value, in order. It first replaced X by Y, and THEN replaced Y by A ref. So the constraint goes from (X, Y) to (Y, Y) to (A ref, A ref). The problem comes from the fact that the reification is not "atomic". We'd want to simultaneously replace all type variables.

It then checks is_subtype_constraint((Y, A ref), (A ref, A ref)), which fails with

Y #any is not a subtype of A ref: the subtype has no constraint

nice triaging @plietar

Based on that triaging, I'm marking as difficulty: hard.