Lookup fixities for reexports without subordinates (#642)

So we agree that reexported declarations which do not have subordinates (for example top-level functions) shouldn't have gotten fixities reexported according to the current logic. I wondered why for example Prelude.($) which is obviously reexported from GHC.Base has fixities attached (c.f. http://hackage.haskell.org/package/base-4.9.1.0/docs/Prelude.html#v:-36-).

The reason is this: In mkMaps we lookup all the subordinates of top-level declarations, of course top-level functions don't have subordinates so for them the resulting list is empty. In #644 I established the invariant that there won't be any empty lists in the subordinate map. Without the patch from #642 top-level functions now started to fail reexporting their fixities.

haddock-api/src/Haddock/Interface/AttachInstances.hs

@@ -18,6 +18,7 @@ import Haddock.Types
 import Haddock.Convert
 import Haddock.GhcUtils
 
+import Control.Applicative
 import Control.Arrow hiding ((<+>))
 import Data.List
 import Data.Ord (comparing)
@@ -119,7 +120,7 @@ attachToExportItem index expInfo iface ifaceMap instIfaceMap export =
                                } = e { expItemFixities =
       nubByName fst $ expItemFixities e ++
       [ (n',f) | n <- getMainDeclBinder d
-              , Just subs <- [instLookup instSubMap n iface ifaceMap instIfaceMap]
+              , Just subs <- [instLookup instSubMap n iface ifaceMap instIfaceMap <|> Just []]
               , n' <- n : (subs ++ patsyn_names)
               , Just f <- [instLookup instFixMap n' iface ifaceMap instIfaceMap]
       ] }