Another major improvement of "improvement"
I wasn't very happy with my fix to Trac #10009. This is much better. The main idea is that the inert set now contains a "model", which embodies *all* the (nominal) equalities that we know about, with a view to exposing unifications. This requires a lot fewer iterations of the solver than before. There are extensive comments in TcSMonad: Note [inert_model: the inert model] Note [Adding an inert canonical constraint the InertCans] The big changes are * New inert_model field in InertCans * Functions addInertEq, addInertCan deal with adding a constraint, maintaining the model * A nice improvement is that unification variables can unify with fmvs, so that from, say alpha ~ fmv we get alpha := fmv See Note [Orientation of equalities with fmvs] in TcFlatten It's still not perfect, as the Note explains New flag -fconstraint-solver-iterations=n, allows us to control the number of constraint solver iterations, and in particular will flag up when it's more than a small number. Performance is generally slightly better: T5837 is a lot better for some reason.