[GHC] #15691: Marking Pred(S n) = n as injective

[GHC]

#15691: Marking Pred(S n) = n as injective
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.6.1
  (Type checker)                     |
           Keywords:  TypeFamilies,  |  Operating System:  Unknown/Multiple
  InjectiveFamilies                  |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Should `Pred` be injective? Please close the ticket if this is a known
 limitation

 {{{#!hs
 {-# Language DataKinds, TypeFamilyDependencies #-}

 data N = O | S N

 type family
  Pred n = res | res -> n where
  Pred(S n) = n
 }}}

 fails with

 {{{
     • Type family equation violates injectivity annotation.
       RHS of injective type family equation is a bare type variable
       but these LHS type and kind patterns are not bare variables: ‘'S n’
         Pred ('S n) = n -- Defined at 462.hs:7:2
     • In the equations for closed type family ‘Pred’
       In the type family declaration for ‘Pred’
   |
 7 |  Pred(S n) = n
   |  ^^^^^^^^^^^^^
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15691>
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