unexpected difference between newtype and data

The following code compiles fine

-- | A referable class
-- An object can only have one reference.
-- For example, a function is either referenced by name of by signature but not both.
class ( Eq (Ref a)
      , Ord (Ref a)
      , Hashable (Ref a)
      , Show (Ref a)
      , Read (Ref a)
      , Data (Ref a)
      ) => Referable a where
    type Ref a
    toRef :: a -> Ref a

data Referable a => 
            RefMap a = RefMap { -- | the HashMap
                                toHashMap :: HashMap.Map (Ref a) a
                              } deriving (Eq, Ord, Show, Read, Data)

yet following the hlint hint to replace data by the equivalent newtype results in the following issues:

  src\TorXakis\Referable.hs:59:43: error:
        * Could not deduce (Eq (Ref a))
            arising from the 'deriving' clause of a data type declaration
          from the context: Eq a
            bound by the deriving clause for `Eq (RefMap a)'
            at src\TorXakis\Referable.hs:59:43-44
          Possible fix:
            use a standalone 'deriving instance' declaration,
              so you can specify the instance context yourself
        * When deriving the instance for (Eq (RefMap a))
       |
    59 |                               } deriving (Eq, Ord, Show, Read, Data)
       |                                           ^^

    src\TorXakis\Referable.hs:59:47: error:
        * Could not deduce (Ord (Ref a))
            arising from the 'deriving' clause of a data type declaration
          from the context: Ord a
            bound by the deriving clause for `Ord (RefMap a)'
            at src\TorXakis\Referable.hs:59:47-49
          Possible fix:
            use a standalone 'deriving instance' declaration,
              so you can specify the instance context yourself
        * When deriving the instance for (Ord (RefMap a))
       |
    59 |                               } deriving (Eq, Ord, Show, Read, Data)

I assume this is a bug...

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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