deriving Data.Typeable on phantom newtypes does not report that DeriveDataTypeable is needed

[guest]

newtype Foo x = Foo deriving (Typeable)

reports the rather cryptic error message:

Can't make a derived instance of `Typeable (Foo x)'

(even with cunning newtype deriving:)

In the newtype declaration for `Foo'

When newtype is replaced with data, this much more useful error message is given:

Can't make a derived instance of `Typeable (Foo x)'

(You need -XDeriveDataTypeable to derive an instance for this class)

In the data type declaration for `Foo'

[simonpj]

Thanks for the suggestion. Happily this is already done: see #2604.

Simon