Instance constraints should be used when deriving on associated data types

Consider this program:

{-# LANGUAGE TypeFamilies, FlexibleContexts #-}

class Eq (Associated a) => Foo a where
    data Associated a

instance Foo a => Foo (Maybe a) where
    data Associated (Maybe a) = AssociatedMaybe (Associated a)
                              deriving (Eq)

This does not compile, giving this error message:

/Users/mbolingbroke/Junk/Repro.hs:9:40:
    No instance for (Eq (Associated a))
      arising from the 'deriving' clause of a data type declaration
                   at /Users/mbolingbroke/Junk/Repro.hs:9:40-41
    Possible fix:
      add an instance declaration for (Eq (Associated a))
      or use a standalone 'deriving instance' declaration instead,
         so you can specify the instance context yourself
    When deriving the instance for (Eq (Associated (Maybe a)))

However, this is surprising because I clearly state that a is Foo, and hence (Associated a) has an Eq instance by the superclass constraint on Foo.

If I point this out explicitly using standalone deriving it works:

{-# LANGUAGE TypeFamilies, FlexibleContexts #-}
{-# LANGUAGE StandaloneDeriving, FlexibleInstances #-}

class Eq (Associated a) => Foo a where
    data Associated a

instance Foo a => Foo (Maybe a) where
    data Associated (Maybe a) = AssociatedMaybe (Associated a)
--                              deriving (Eq)

deriving instance Foo a => Eq (Associated (Maybe a))

So I think the default behaviour for "deriving" on an associated data family should be to include the constraints from the enclosing instance. For now the workaround is just to use standalone deriving.

Trac metadata

Trac field	Value
Version	6.12.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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