UndecidableSuperClasses causes the compiler to spin with UndecidableInstances

Looks like I spoke too soon when I said all my examples worked in #10318 (closed) -- it doesn't seem to work when the superclass cycle gets sufficiently interesting, possibly caused by the use of PolyKinds in the style mentioned in #9201 (closed).

I took my hask code, and removed the shimming hacks above, and the following stripped down example sends the compiler into an infinite loop, which I believe should be able to work:

{-# language KindSignatures, PolyKinds, TypeFamilies, 
  NoImplicitPrelude, FlexibleContexts,
  MultiParamTypeClasses, GADTs, 
  ConstraintKinds, FlexibleInstances, 
  FunctionalDependencies, UndecidableSuperClasses #-}

import GHC.Types (Constraint)
import qualified Prelude

data Nat (c :: i -> i -> *) (d :: j -> j -> *) (f :: i -> j) (g :: i -> j)

class Functor p (Nat p (->)) p => Category (p :: i -> i -> *)
class (Category dom, Category cod) => Functor (dom :: i -> i -> *) (cod :: j -> j -> *) (f :: i -> j) | f -> dom cod

instance (Category c, Category d) => Category (Nat c d)
instance (Category c, Category d) => Functor (Nat c d) (Nat (Nat c d) (->)) (Nat c d)
instance (Category c, Category d) => Functor (Nat c d) (->) (Nat c d f)

instance Category (->)
instance Functor (->) (->) ((->) e)
instance Functor (->) (Nat (->) (->)) (->)

Sorry for the largish example, but I don't know how to strip it down smaller than the 6 instances that remain.

One potentially telling observation is that without the instances it compiles, and produces what I expect, so the UndecidableSuperClasses part seems to be letting the classes compile, but there seems to be a bad interaction with the way the instances work.

Also, in this stripped down form, I can remove the use of UndecidableInstances and that avoids the spinning problem, but once I flesh it out further I need UndecidableInstances in the "real" version of the problem.

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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