[GHC] #10338: GHC Forgets Constraints

[GHC]

#10338: GHC Forgets Constraints
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        Reporter:  crockeea          |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.3
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by crockeea):

 I've got a new test case for what I assume is this bug. Unfortunately, I
 can't find a workaround in this case.

 {{{#!hs
 {-# LANGUAGE MultiParamTypeClasses, ScopedTypeVariables,
              ConstraintKinds, KindSignatures, TypeFamilies,
              FlexibleInstances #-}

 import GHC.Prim
 import Control.Monad

 class Unsat (a :: * -> *) c

 class Qux (t :: * -> *) where

   type QCtx t q :: Constraint
   qux :: (QCtx t q, Monad rnd)
                => v -> rnd (t q)

 instance Qux t where
   type QCtx t q = (Unsat t q)

 class (Qux c) => C (c :: * -> *) r

 mymap :: c Double -> c i
 mymap = undefined

 foo :: forall c z v rnd . (C c z, Monad rnd, Num z) => v -> rnd (c z)
 foo svar = liftM mymap $ qux svar

 I've tried a wide array of explicit type sigs on `foo`, but nothing seems
 to make it find the dictionary for `Qux` inherited from `C`. I really need
 a fix for this.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10338#comment:4>
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