[GHC] #15305: Erroneous "non-exhaustive pattern match" using nested GADT with strictness annotation

[GHC]

#15305: Erroneous "non-exhaustive pattern match" using nested GADT with strictness
annotation
-------------------------------------+-------------------------------------
           Reporter:  jkoppel        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 In the following code, `fun` contains an exhaustive pattern match, but,
 when compiling with -Wall, ghc erroneously reports a non-exhaustive
 pattern match.

 {{{#!hs

 data (:+:) f g a = Inl !(f a) | Inr !(g a)

 data A
 data B

 data Foo l where
   Foo :: Foo A

 data Bar l where
   Bar :: Bar B

 type Sig = Foo :+: Bar

 fun :: Sig B -> Int
 fun (Inr Bar) = 1

 }}}

 This report came from https://stackoverflow.com/questions/16225281/gadts-
 failed-exhaustiveness-checking . Without strictness annotations, this is
 indeed a failed exhaustive check, due to bottom. I spoke to Richard
 Eisenberg at PLDI a few days ago, and he informed me that, if this warning
 did not disappear with strictness annotations, it was a bug. I added
 strictness annotations, and it did not disappear. I've tried all
 combinations of using strictness annotations and/or running with `{-#
 LANGUAGE Strict #-}`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15305>
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