Deriving Data at higher kinds

Here's what Lennart Spitzner wanted to do:

> {-# LANGUAGE StandaloneDeriving #-}
> {-# LANGUAGE DeriveDataTypeable #-}
> {-# LANGUAGE FlexibleInstances #-}
> 
> import Data.Data ( Data )
>
> data Foo f = Foo (f Bool) (f Int)
> 
> deriving instance Data (Foo [])
> deriving instance Data (Foo Maybe)

Of course you can't derive Data for Foo because we don't know what f is, so Lennart is making multiple instances, one for each instance of f. It's a bit clumsy. What we would really like is

deriving instance (forall a. Data  => Data (f a)) => Data (Foo f)

but we don't have higher order instances yet! So Lennart is manually making two instances.

This should work, but he gets

> Main.hs: line 45, column 1:
>   Multiple declarations of ‘$cr2C’
>     Declared at: Main.hs:44:1
>                  Main.hs:45:1
> Main.hs: line 45, column 1:
>   Multiple declarations of ‘$tr2D’
>     Declared at: Main.hs:44:1
>                  Main.hs:45:1

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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