[GHC] #10835: Regression in standalone Data deriving for phantom types

3 Sep 2015

      #10835: Regression in standalone Data deriving for phantom types
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              Reporter:  Feuerbach   |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.2
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 Thise code

 {{{#!hs
 {-# LANGUAGE DeriveDataTypeable, StandaloneDeriving, PolyKinds #-}
 module M where
 import Data.Data
 import Data.Typeable

 data Phantom t = Phantom
   deriving Typeable
 deriving instance Typeable t => Data (Phantom t)

 }}}

 compiles with 7.8.4 but not with 7.10.2. The error is

 {{{
 data.hs:8:1:
     Could not deduce (Typeable Phantom)
       arising from the superclasses of an instance declaration
     from the context (Typeable t)
       bound by the instance declaration at data.hs:8:1-48
     In the instance declaration for ‘Data (Phantom t)’
 }}}

 PolyKinds here is a hack I came up with to make this compile with 7.8; but
 ideally, this code should compile whether PolyKinds is enabled or not.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10835
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