[GHC] #12163: Incorrect error message when deriving Functor for a GADT

[GHC]

#12163: Incorrect error message when deriving Functor for a GADT
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        Reporter:  benjamin.hodgson  |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  MacOS X           |         Architecture:  x86_64
 Type of failure:  Incorrect         |  (amd64)
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by benjamin.hodgson:

@@ -20,1 +20,2 @@
- deriving declaration works though.
+ deriving declaration works though. I'm guessing GHC is turning my
+ declaration into `Mk :: (a ~ Int) => a -> b -> T a b`?

New description:

 {{{#!hs
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE GADTs #-}

 data T a b where
     Mk :: Int -> b -> T Int b
     deriving (Functor)
 }}}

 GHC says:
 {{{
     Can't make a derived instance of ‘Functor (T a)’:
       Constructor ‘Mk’ has existentials or constraints in its type
       Possible fix: use a standalone deriving declaration instead
     In the data declaration for ‘T’
 }}}

 This is not true - Mk does not existentially quantify anything, nor does
 it have a constraint in its type. The suggested fix of using a standalone
 deriving declaration works though. I'm guessing GHC is turning my
 declaration into `Mk :: (a ~ Int) => a -> b -> T a b`?

 Tested on 7.10.3 running in OSX Mavericks.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12163#comment:2>
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