[GHC] #10598: DeriveAnyClass and GND don't work well together

2 Jul 2015

      ...
In case you try to derive some class on a newtype, and
 -XGeneralizedNewtypeDeriving is also on, -XDeriveAnyClass takes
#10598: DeriveAnyClass and GND don't work well together
-------------------------------------+-------------------------------------
              Reporter:  osa1        |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.11
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 I think we definitely have a bug here, but I'm not sure what it really is.

 Here's the program:

 {{{
 newtype MyMaybe a = MyMaybe (Maybe a)
   deriving (Functor, Show)

 main = print $ MyMaybe $ Just (10 :: Int)
 }}}

 I'm using GHC 7.10.1.

 {{{
 ➜  deriveany_bug  ghc --make -fforce-recomp Test.hs -XDeriveAnyClass
 -XGeneralizedNewtypeDeriving
 [1 of 1] Compiling Main             ( Test.hs, Test.o )

 Test.hs:2:13:
     Can't make a derived instance of ‘Functor MyMaybe’
       (even with cunning newtype deriving):
       You need DeriveFunctor to derive an instance for this class
       Try GeneralizedNewtypeDeriving for GHC's newtype-deriving extension
     In the newtype declaration for ‘MyMaybe’
 }}}

 Just to try, changing argument order:

 {{{
 ➜  deriveany_bug  ghc --make -fforce-recomp Test.hs
 -XGeneralizedNewtypeDeriving -XDeriveAnyClass
 [1 of 1] Compiling Main             ( Test.hs, Test.o )

 Test.hs:2:13:
     Can't make a derived instance of ‘Functor MyMaybe’
       (even with cunning newtype deriving):
       You need DeriveFunctor to derive an instance for this class
       Try GeneralizedNewtypeDeriving for GHC's newtype-deriving extension
     In the newtype declaration for ‘MyMaybe’
 }}}

 It works fine if I remove `DeriveAnyClass`:

 {{{
 ➜  deriveany_bug  ghc --make -fforce-recomp Test.hs
 -XGeneralizedNewtypeDeriving
 [1 of 1] Compiling Main             ( Test.hs, Test.o )
 Linking Test ...
 }}}

 GHC HEAD is failing in exactly the same way.

 User manual is saying this in 7.5.6:

 precedence.

 But then why is it telling me to enable `GeneralizedNewtypeDeriving` in
 the error message? Even if I already enabled it?

 Also, maybe it could try `GND` when `DeriveAnyClass` fails? Because the
 doc is saying `DeriveAnyClass` has precedence but doesn't specify what
 happens if it fails.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10598
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