[GHC] #12919: Equality not used for substitution

[GHC]

#12919: Equality not used for substitution
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           Reporter:  int-index      |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           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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 This code

 {{{
 {-# LANGUAGE TypeInType, TypeFamilies, GADTs, ConstraintKinds #-}

 import Data.Kind

 data N = Z

 data V :: N -> Type where
   VZ :: V Z

 type family VC (n :: N) :: Type where
   VC Z = Type

 type family VF (xs :: V n) (f :: VC n) :: Type where
   VF VZ f = f

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

 prop :: xs ~ VZ => Dict (VF xs f ~ f)
 prop = Dict
 }}}

 fails with this error:

 {{{
 Op.hs:20:8: error:
     • Couldn't match type ‘f’ with ‘VF 'VZ f’
         arising from a use of ‘Dict’
       ‘f’ is a rigid type variable bound by
         the type signature for:
           prop :: forall (xs :: V 'Z) f. xs ~ 'VZ => Dict VF xs f ~ f
         at Op.hs:19:9
     • In the expression: Dict
       In an equation for ‘prop’: prop = Dict
     • Relevant bindings include
         prop :: Dict VF xs f ~ f (bound at Op.hs:20:1)
 }}}

 However, if I substitute `xs` with `VZ` (by hand) in the type of `prop`,
 it compiles. Can be reproduced with GHC 8.0.1 but not HEAD.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12919>
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