Preserve docs on type family instances (#867)

* Preserve docs on type family instances

The only problem was that the instance location was slightly off
for type family instances.

* Accept output

(cherry picked from commit 133e9c2c168db19c1135479f7ab144c4e33af2a4)

1 files changed, 5 insertions, 2 deletions

diff --git a/haddock-api/src/Haddock/Interface/Create.hs b/haddock-api/src/Haddock/Interface/Create.hs
index 78242990..c4df2090 100644
--- a/haddock-api/src/Haddock/Interface/Create.hs
+++ b/haddock-api/src/Haddock/Interface/Create.hs
@@ -419,9 +419,12 @@ mkMaps dflags pkgName gre instances decls = do
     instanceMap = M.fromList [ (getSrcSpan n, n) | n <- instances ]
 
     names :: SrcSpan -> HsDecl GhcRn -> [Name]
-    names l (InstD _ d) = maybeToList (M.lookup loc instanceMap) -- See note [2].
+    names _ (InstD _ d) = maybeToList (M.lookup loc instanceMap) -- See note [2].
       where loc = case d of
-              TyFamInstD _ _ -> l -- The CoAx's loc is the whole line, but only for TFs
+              -- The CoAx's loc is the whole line, but only for TFs. The
+              -- workaround is to dig into the family instance declaration and
+              -- get the identifier with the right location.
+              TyFamInstD _ (TyFamInstDecl d') -> getLoc (feqn_tycon (hsib_body d'))
               _ -> getInstLoc d
     names l (DerivD {}) = maybeToList (M.lookup l instanceMap) -- See note [2].
     names _ decl = getMainDeclBinder decl