[GHC] #12526: regression in type inference with respect to type families

[GHC]

#12526: regression in type inference with respect to type families
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        Reporter:  Lemming           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.2
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  10634             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Here is a yet simpler version of the problem
 {{{
 {-# LANGUAGE TypeFamilies, MonoLocalBinds #-}
 module T12526 where

 type family P (s :: * -> *) :: * -> * -> *
 type instance P Signal = Causal

 type family S (p :: * -> * -> *) :: * -> *
 type instance S Causal = Signal

 class (P (S p) ~ p) => CP p
 instance CP Causal

 data Signal a = Signal
 data Causal a b = Causal

 shapeModOsci :: CP p => p Float Float
 shapeModOsci = undefined

 f :: Causal Float Float -> Bool
 f = undefined

 -- This fails
 ping :: Bool
 ping = let osci = shapeModOsci in  f osci


 -- This works
 -- ping :: Bool
 -- ping = f shapeModOsci
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12526#comment:9>
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