Solver hits iteration limit in code without recursive constraints
This example (derived from `xmonad-contrib`) failed to compile with `master`,
```hs
{-# 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.
issue