Data instance for Data.Map is incomplete

There's no dataCast1 defined for Data.Map. This makes the ext1 family unusable with Maps, since ext is built on dataCast rather than gcast.

Observe:

Prelude Data.Data Data.Generics Data.Map> gcast1 (Just empty :: forall d. (Data d) => Maybe (Map Int d)) :: Maybe (Maybe (Map Int Int))
Just (Just (fromList []))
Prelude Data.Data Data.Generics Data.Map> dataCast1 (Just empty :: forall d. (Data d) => Maybe (Map Int d)) :: Maybe (Maybe (Map Int Int))
Nothing

Trac metadata

Trac field	Value
Version	7.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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