[GHC] #8705: Type inference regression with local dictionaries

[GHC]

#8705: Type inference regression with local dictionaries
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        Reporter:  goldfire                 |            Owner:
            Type:  bug                      |           Status:  new
        Priority:  high                     |        Milestone:  7.8.1
       Component:  Compiler (Type checker)  |          Version:  7.8.1-rc1
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Comment (by goldfire):

 Here is a version without the `singletons` dependency:

 {{{
 {-# LANGUAGE TypeOperators, DataKinds, PolyKinds,
              MultiParamTypeClasses, GADTs, ConstraintKinds, TypeFamilies
 #-}

 module Bug where

 data family Sing (a :: k)
 data Proxy a = Proxy

 data instance Sing (a :: Maybe k) where
   SJust :: Sing h -> Sing (Just h)

 data Dict c where
   Dict :: c => Dict c

 -- A less-than-or-equal relation among naturals
 class a :<=: b

 sLeq :: Sing n -> Sing n2 -> Dict (n :<=: n2)
 sLeq = undefined

 insert_ascending :: (lst ~ Just n1) => Proxy n1 -> Sing n -> Sing lst ->
 Dict (n :<=: n1)
 insert_ascending _ n (SJust h) =
   case sLeq n h of
     Dict -> Dict
 }}}

 Interestingly, when I tried getting rid of the data family, the code
 started compiling. Perhaps there's a problem with normalisation somewhere?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8705#comment:5>
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