[GHC] #11754: Error in optCoercion

[GHC]

#11754: Error in optCoercion
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           Reporter:  simonpj        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This program fails Lint after a run of the simplifier.
 {{{
 {-# LANGUAGE TypeOperators, UndecidableSuperClasses, KindSignatures,
 TypeFamilies, FlexibleContexts #-}

 module T11728a where

 import Data.Kind
 import Data.Void

 newtype K a x = K a
 newtype I   x = I x

 data (f + g) x = L (f x) | R (g x)
 data (f × g) x = f x :×: g x

 class Differentiable (D f) => Differentiable f where
   type D (f :: Type -> Type) :: Type -> Type

 instance Differentiable (K a) where
   type D (K a) = K Void

 instance Differentiable I where
   type D I = K ()

 instance (Differentiable f₁, Differentiable f₂) => Differentiable (f₁ +
 f₂) where
   type D (f₁ + f₂) = D f₁ + D f₂

 instance (Differentiable f₁, Differentiable f₂) => Differentiable (f₁ ×
 f₂) where
   type D (f₁ × f₂) = (D f₁ × f₂) + (f₁ × D f₂)
 }}}
 Originally reported in #11728, but it's a totally different problem.

 Richard has nailed it already... patch coming from him.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11754>
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