[GHC] #11984: Pattern match incompleteness / inaccessibility discrepancy

[GHC]

#11984: Pattern match incompleteness / inaccessibility discrepancy
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           Reporter:  goldfire       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           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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 Consider this module:

 {{{
 {-# LANGUAGE PolyKinds, TypeOperators, DataKinds, TypeFamilies, GADTs #-}

 module Bug where

 data family Sing (a :: k)

 data Schema = Sch [Bool]

 data instance Sing (x :: Schema) where
   SSch :: Sing x -> Sing ('Sch x)

 data instance Sing (x :: [k]) where
   SNil :: Sing '[]
   SCons :: Sing a -> Sing b -> Sing (a ': b)

 data G a where
   GCons :: G ('Sch (a ': b))

 eval :: G s -> Sing s -> ()
 eval GCons s =
         case s of
           -- SSch SNil -> undefined
           SSch (SCons _ _) -> undefined
 }}}

 Upon seeing this, GHC says

 {{{
 Bug.hs:21:9: warning: [-Wincomplete-patterns]
     Pattern match(es) are non-exhaustive
     In a case alternative: Patterns not matched: (SSch SNil)
 }}}

 So I uncomment the second-to-last line, inducing GHC to say

 {{{
 Bug.hs:22:16: error:
     • Couldn't match type ‘a : b’ with ‘'[]’
       Inaccessible code in
         a pattern with constructor: SNil :: forall k. Sing '[],
         in a case alternative
     • In the pattern: SNil
       In the pattern: SSch SNil
       In a case alternative: SSch SNil -> undefined
 }}}

 Thankfully, this pattern is much rarer than it once was, but it's a bit
 sad that it's still possible.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11984>
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