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

[GHC]

#10598: DeriveAnyClass and GND don't work well together
-------------------------------------+-------------------------------------
        Reporter:  osa1              |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
Description changed by osa1:

Old description:

> 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:
>
> > In case you try to derive some class on a newtype, and
> -XGeneralizedNewtypeDeriving is also on, -XDeriveAnyClass takes
> 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.

New description:

 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:

 > In case you try to derive some class on a newtype, and
 -XGeneralizedNewtypeDeriving is also on, -XDeriveAnyClass takes
 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.

 EDIT: I'd like to work on this myself if experts here help me figuring the
 right behavior here.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10598#comment:1>
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