Deriving for H98 types using GADT syntax fails

This example from the GADT section of the user manual:

data Maybe1 a where {
      Nothing1 :: Maybe1 a ;
      Just1    :: a -> Maybe1 a
    } deriving (Eq,Ord)

Fails with:

Test_show.hs:28:5:
    Can't derive instances where the instance context mentions
    type variables that are not data type parameters
      Offending constraint: Eq a
    When deriving the instance for `Eq (Maybe1 a)'

Test_show.hs:28:5:
    Variable occurs more often in a constraint than in the instance head
      in the constraint: Eq a
    (Use -fallow-undecidable-instances to permit this)
    In the derived instance declaration
      instance (Eq a) => Eq (Maybe1 a) 

-dppr-debug gives this:

Test_show.hs:28:5:
    Can't derive instances where the instance context mentions
    type variables that are not data type parameters
      Offending constraint: base:GHC.Base.Eq{tc 23} a{tv adf} [tv]
    When deriving the instance for `base:GHC.Base.Eq{tc 23} (main:Test_show.Maybe1{tc ray} a{tv adg} [tv])'

Test_show.hs:28:5:
    Variable occurs more often in a constraint than in the instance head
      in the constraint: base:GHC.Base.Eq{tc 23} a{tv adf} [tv]
    (Use -fallow-undecidable-instances to permit this)
    In the derived instance declaration
      instance (base:GHC.Base.Eq{tc 23} a{tv adf} [tv]) =>
               base:GHC.Base.Eq{tc 23} (main:Test_show.Maybe1{tc ray} a{tv adg} [tv])

The two "a":s appear to be different, "a{tv adf}" and "a{tv adg}".

Trac metadata

Trac field	Value
Version	6.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown