[GHC] #13345: GHC 8 type checker regression

26 Feb 2017


      #13345: GHC 8 type checker regression
-------------------------------------+-------------------------------------
           Reporter:  andreas.abel   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
  (Type checker)                     |
           Keywords:  type           |  Operating System:  Unknown/Multiple
  annotation                         |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 GHC 8.0.1/2 needs a type annotation where GHC 7.8 can do without:
 {{{#!hs
 {-# 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'
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13345
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler