Newtype deriving error messages coming out too late

[simonpj]

Consider this

newtype Ego a = Ego a deriving (Ord)

f :: Ord a => Ego a -> Ego a -> Bool
f e1 e2 = e1 < e2

GHC 6.7 reports an error like this:

    Could not deduce (Eq (Ego a)) from the context (Ord a)
      arising from a use of `<' at Foo10.hs:6:10-16
    In the expression: e1 < e2
    In the definition of `f': f e1 e2 = e1 < e2

This error should have come when you gave the instance decl. The reason is that GHC's newtype deriving is generating an instance like

instance Eq (Ego a) => Ord (Ego a)

It would make more sense to report the missing instance for Eq right there, rather than abstracting over it.

Thanks to Dougal Stanton for pointing this out:  http://www.haskell.org/pipermail/haskell-cafe/2007-August/030224.html

[simonpj]

And indeed this is just what GHC (6.8 and HEAD) now does:

Der.hs:3:0:
    No instance for (Eq (Ego a))
      arising from the 'deriving' clause of a data type declaration
		   at Der.hs:3:0-35
    Possible fix: add an instance declaration for (Eq (Ego a))
    When deriving the instance for (Ord (Ego a))

So it's fixed, with no merging to be done. I've added a test case to the test suite though.