From 026de54b89f5018c9a29ac3494686cd86ea845df Mon Sep 17 00:00:00 2001
From: "simonpj@microsoft.com" <unknown>
Date: Mon, 21 Dec 2009 15:55:09 +0000
Subject: [PATCH] Fix Trac #3776

An easy fix.  See Note [Usage for sub-bndrs] in RnEnv.
---
 compiler/rename/RnEnv.lhs |   26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/compiler/rename/RnEnv.lhs b/compiler/rename/RnEnv.lhs
index c81d701..2f5743a 100644
--- a/compiler/rename/RnEnv.lhs
+++ b/compiler/rename/RnEnv.lhs
@@ -284,12 +284,12 @@ lookupSubBndr parent doc rdr_name
   | otherwise	-- Find all the things the rdr-name maps to
   = do	{	-- and pick the one with the right parent name
 	; env <- getGlobalRdrEnv
-        ; let gres = (lookupGlobalRdrEnv env (rdrNameOcc rdr_name))
+        ; let gres = lookupGlobalRdrEnv env (rdrNameOcc rdr_name)
 	; case pick parent gres  of
 		-- NB: lookupGlobalRdrEnv, not lookupGRE_RdrName!
 		--     The latter does pickGREs, but we want to allow 'x'
 		--     even if only 'M.x' is in scope
-	    [gre] -> do { addUsedRdrName gre rdr_name
+	    [gre] -> do { addUsedRdrNames (used_rdr_names gre)
                         ; return (gre_name gre) }
 	    []    -> do { addErr (unknownSubordinateErr doc rdr_name)
 			; traceRn (text "RnEnv.lookup_sub_bndr" <+> (ppr rdr_name $$ ppr gres))
@@ -306,6 +306,15 @@ lookupSubBndr parent doc rdr_name
     right_parent p (GRE { gre_par = ParentIs p' }) = p==p' 
     right_parent _ _                               = False
 
+    -- Note [Usage for sub-bndrs]
+    used_rdr_names gre
+      | isQual rdr_name = [rdr_name]
+      | otherwise       = case gre_prov gre of
+                            LocalDef -> [rdr_name]
+			    Imported is -> map mk_qual_rdr is
+    mk_qual_rdr imp_spec = mkRdrQual (is_as (is_decl imp_spec)) rdr_occ
+    rdr_occ = rdrNameOcc rdr_name    
+
 newIPNameRn :: IPName RdrName -> TcRnIf m n (IPName Name)
 newIPNameRn ip_rdr = newIPName (mapIPName rdrNameOcc ip_rdr)
 
@@ -320,12 +329,25 @@ lookupFamInstDeclBndr tyclGroupEnv (L loc rdr_name)
         (gre:_) -> return $ gre_name gre
           -- if there is more than one, an error will be raised elsewhere
         []      -> lookupOccRn rdr_name
+\end{code}
 
+Note [Usage for sub-bndrs]
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+If you have this
+   import qualified M( C( f ) ) 
+   intance M.C T where
+     f x = x
+then is the qualified import M.f used?  Obviously yes.
+But the RdrName used in the instance decl is unqualified.  In effect,
+we fill in the qualification by looking for f's whose class is M.C
+But when adding to the UsedRdrNames we must make that qualification
+explicit, otherwise we get "Redundant import of M.C".
 
 --------------------------------------------------
 --		Occurrences
 --------------------------------------------------
 
+\begin{code}
 getLookupOccRn :: RnM (Name -> Maybe Name)
 getLookupOccRn
   = getLocalRdrEnv			`thenM` \ local_env ->
-- 
1.7.10.4