Haddock renders imported data types with polymorphic return kinds incorrectly

[RyanGlScott]

(Originally discovered in well-typed/generics-sop#92).

Run Haddock on this file:

module Bug (Product(..)) where

import Data.Functor.Product

And you'll get this:

That return kind is entirely wrong, given that we're also pretty-printing the type variable binders for Product itself. I was under the false impression that I had already fixed this issue in #702, but now that I look more carefully at the code that I added:

haddock/haddock-api/src/Haddock/Convert.hs

Lines 275 to 280 in 44169f4

	synifyDataTyConReturnKind :: TyCon -> Maybe (LHsKind GhcRn)
	synifyDataTyConReturnKind tc
	= case splitFunTys (tyConKind tc) of
	(_, ret_kind)
	| isLiftedTypeKind ret_kind -> Nothing -- Don't bother displaying :: *
	| otherwise -> Just (synifyKindSig ret_kind)

It turns out my solution wasn't sufficiently general. We're using splitFunTys here, but that won't work in the case where the return kind is headed by a forall (such as in the example above). A more robust solution is to use dropForAlls followed by splitFunTys.

Patch incoming.

[harpocrates]

Thanks for the bug report, accompanied as usual with a great test case. 😄