[GHC] #14744: Non-exhaustive patterns in case in GHCi with quantified class contexts

31 Jan 2018

      #14744: Non-exhaustive patterns in case in GHCi with quantified class contexts
-------------------------------------+-------------------------------------
           Reporter:  kosmikus       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.5
           Keywords:  wipT2893,      |  Operating System:  Unknown/Multiple
  QuantifiedConstraints              |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This appears in the wip/T2893 branch.

 To reproduce, create the following module
 {{{
 {-# LANGUAGE ConstraintKinds, GADTs, KindSignatures, QuantifiedConstraints
 #-}
 module Bug where

 import GHC.Exts

 data Foo :: (* -> Constraint) -> * where
   MkFoo :: (forall a . c a => c (f a), c a) => (f a) -> Foo c
 }}}
 and load it into GHCi:
 {{{
 Bug> foo = MkFoo "foo" :: Foo Show
 *** Exception: compiler/typecheck/TcType.hs:(1965,9)-(1985,76): Non-
 exhaustive patterns in case
 }}}

 Putting the same declaration into the module itself does not seem to
 trigger the bug.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14744
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