ghc-8.0.2+ rejects more instance declarations

Sorry for the vague title.

For a program (sorry again but I failed to shrink it further):

{-# language ExistentialQuantification, KindSignatures, DataKinds, GADTs, UndecidableInstances, AllowAmbiguousTypes #-}

class Pretty a where
  pretty :: a -> String
  prettyList :: [a] -> String
  prettyList = concatMap pretty

data Def e = forall t. FunDef (Type t) (e t)

data Nat = Z | S Nat

data F (t :: Nat) where
  FZ :: F (S Z)
  FS :: F n -> F (S n)

data Type (t :: Nat) = TVar (F t)

data Expr (t :: Nat) e = Fun (Type t)
                       | App (e t) (e t)

data TE t = TE (Type t) (Expr t TE)

-- all methods left undefined, as they're insignificant
instance Pretty t => Pretty [t] where
  pretty = undefined
instance (Pretty (e t)) => Pretty (Def e) where
  pretty (FunDef t e) = undefined
instance Pretty (Type t) where
  pretty (TVar t) = undefined
instance Pretty (e t) => Pretty (Expr t e) where
  pretty (Fun t) = undefined
  pretty (App e1 e2) = undefined
instance Pretty (TE t) where
  pretty (TE t e) = undefined

ghc-8.0.1 is happy with it, whereas later versions (tested with 8.0.2 and 8.2.1-rc2) reject it. The error msg is:

GHCInstance1.hs:27:10: error:
    • Could not deduce (Pretty (e t0))
        arising from a use of ‘Main.$dmprettyList’
      from the context: Pretty (e t)
        bound by the instance declaration at GHCInstance1.hs:27:10-41
      The type variable ‘t0’ is ambiguous
      Relevant bindings include
        prettyList :: [Def e] -> String (bound at GHCInstance1.hs:27:10)
    • In the expression: Main.$dmprettyList @Def e
      In an equation for ‘prettyList’:
          prettyList = Main.$dmprettyList @Def e
      In the instance declaration for ‘Pretty (Def e)’

If I comment out the prettyList method from the class definition, then all happy. I'm not sure if it's a bug in 8.0.1 or a regression (and hard to reason about the behaviours). If it was the later, how should I migrate from 8.0.1? The Pretty class is a library (e.g. ansi-wl-pprint) so no way I can change it.

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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