This example (derived from xmonad-contrib) failed to compile with master,

{-# LANGUAGE ExistentialQuantification, RankNTypes, MultiParamTypeClasses,
             FunctionalDependencies, FlexibleInstances, FlexibleContexts #-}

module XMonad.Layout.MultiToggle where

import Data.Typeable

-- This appears to be the culprit
expand :: (HList ts a) => MultiToggleS ts l a -> MultiToggle ts l a
expand (MultiToggleS b ts) =
    resolve ts id
        (\x mt -> let g = transform' x in mt{ currLayout = g $ currLayout mt })
        (MultiToggle (EL b id) ts)

-- Removing the Typeable constraint here allows compilation to finish
class (Typeable t) => Transformer t a | t -> a where
    transform :: t
              -> l a
              -> (forall l'. l' a -> (l' a -> l a) -> b)
              -> b

data  EL l a = forall l'. EL (l' a) (l' a -> l a)

transform' :: (Transformer t a) => t -> EL l a -> EL l a
transform' t (EL l det) = undefined

data MultiToggleS ts l a = MultiToggleS (l a) ts
                         deriving (Read, Show)

data MultiToggle ts l a = MultiToggle{
    currLayout :: EL l a,
    transformers :: ts
}

class HList c a where
    resolve :: c -> b -> (forall t. (Transformer t a) => t -> b) -> b

failing during constraint solving with,,

XMonad/Layout/MultiToggle.hs:1:1: error:
    solveWanteds: too many iterations (limit = 4)
      Unsolved: WC {wc_simple =
                      [D] _ :: Transformer t a (CDictCan)
                      [D] _ :: a_aIoy ~ a (CNonCanonical)
                      [D] _ :: Typeable t (CDictCan)
                    wc_impl =
                      Implic {
                        TcLevel = 7
                        Skolems = (l :: * -> *)
                        No-eqs = True
                        Status = Unsolved
                        Given =
                        Wanted =
                          WC {wc_simple =
                                [W] $dTransformer_aIoM :: Transformer t a (CDictCan)}
                        Binds = Just EvBindsVar<aIoN>
                        the inferred type of g :: EL l_aIoL a_aIoK -> EL l_aIoL a_aIoK }}
      New superclasses found
      Set limit with -fconstraint-solver-iterations=n; n=0 for no limit

Lifting the solver iteration limit just results in a loop.

I suspect the issue may be in the Typeable solving logic, as removing the Typable constraint from Transformer's head allows compilation to proceed.