[GHC] #13931: GHC incorrectly considers type family instances conflicting?

[GHC]

#13931: GHC incorrectly considers type family instances conflicting?
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           Reporter:  vagarenko      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I asked this on stackoverflow first
 https://stackoverflow.com/questions/44958114/why-these-type-family-
 instances-conflicting

 I'm using GHC-8.0.1. This code (requires singletons lib):


 {{{#!hs
 {-# LANGUAGE TypeFamilies            #-}
 {-# LANGUAGE TypeOperators           #-}
 {-# LANGUAGE TypeInType              #-}

 module Bug where

 import Data.Kind
 import Data.Singletons

 type family MkCtx (kx :: Type) (kc :: Type) (c :: kc) (x :: kx) ::
 Constraint
 type instance MkCtx kx (kx ~> Constraint) c x = Apply c x
 type instance MkCtx kx (kx -> Constraint) c x = c x
 }}}

 with the message:

 {{{

 Conflicting family instance declarations:
       forall kx (x :: kx) (c :: kx ~> Constraint).
         MkCtx kx (kx ~> Constraint) c x = Apply c x
       forall kx (x :: kx) (c :: kx -> Constraint).
         MkCtx kx (kx -> Constraint) c x = c x
 }}}

 Why GHC considers these instances conflicting? I don't see how they
 overlap.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13931>
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