[GHC] #13293: ConstrainedClassMethods makes GeneralizedNewtypeDeriving fail

[GHC]

#13293: ConstrainedClassMethods makes GeneralizedNewtypeDeriving fail
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           Reporter:  j.waldmann     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           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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 Example:

 {{{

 {-# language GeneralizedNewtypeDeriving #-}
 {-# language ConstrainedClassMethods #-}

 instance ToDoc Int where toDoc = undefined

 newtype P = P Int deriving ( ToDoc )

 type Doc = ()

 class ToDoc a where
     toDoc :: ToDoc a => a -> Doc
 }}}

 compile with ghc-8.0.2:

 {{{
 C.hs:6:30: error:
     • Couldn't match type ‘Int’ with ‘P’
         arising from the coercion of the method ‘toDoc’
           from type ‘ToDoc Int => Int -> Doc’ to type ‘ToDoc P => P ->
 Doc’
     • When deriving the instance for (ToDoc P)
 }}}

 With ghc-8.0.1, it is fine.

 When I drop the constraint on the method declaration, ghc-8.0.2 compiles
 the program.

 This repetition of the class constraint in the method declaration actually
 came from a copy-paste error, so this issue does not affect my code, but
 it's still puzzling. Repeating the class constraint should not change the
 meaning of the program?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13293>
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