TH derivation doesn't work when the type name is qualified

[neongreen]

Let's try to derive Arbitrary for Maybe, but imported qualified from Prelude:

{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE TemplateHaskell   #-}

import           Data.DeriveTH   (derive, makeArbitrary)
import qualified Prelude         as P

import           Test.QuickCheck (Arbitrary (..))

derive makeArbitrary ''P.Maybe

It won't work:

    Not in scope: type constructor or class ‘Maybe’
    Perhaps you meant ‘P.Maybe’ (imported from Prelude)

    Not in scope: type constructor or class ‘Int’
    Perhaps you meant ‘P.Int’ (imported from Prelude)

When I run GHC with -ddump-splices, I see that derive generates the following code (which doesn't include qualified names):

    derive makeArbitrary ''P.Maybe
  ======>
    instance Arbitrary a_3530822107858468866 =>
             Arbitrary (Maybe a_3530822107858468866) where
      arbitrary
        = do { x <- choose (0 :: Int, 1);
               case x of {
                 0 -> return Nothing
                 1 -> do { x1 <- arbitrary;
                           return (Just x1) }
                 _ -> error "FATAL ERROR: Arbitrary instance, logic bug" } }

[ndmitchell]

Yep, agreed, that's a bug - but potentially a quite large one to fix. As per #6 I'm seeking to pass maintainership on to someone else, so it may be a while before anyone takes a proper look.