GHC 8 type checker regression

GHC 8.0.1/2 needs a type annotation where GHC 7.8 can do without:

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeSynonymInstances, FlexibleInstances, MultiParamTypeClasses #-}

data T = Var | App T T
data S = S

class Tr a b where
  tr :: a -> b
  tr = undefined
  
instance Tr S T where
instance Tr a b => Tr [a] [b] where

test :: [S] -> T
test es = let
    -- es' :: [T]  -- This type annotation is not needed by ghc-7.8
    es' = tr es
  in foldl App Var es'

Trac metadata

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