1 files changed, 48 insertions, 8 deletions

diff --git a/compiler/rename/RnEnv.lhs b/compiler/rename/RnEnv.lhs
index f2a0649f6e..09890181a5 100644
--- a/compiler/rename/RnEnv.lhs
+++ b/compiler/rename/RnEnv.lhs
@@ -223,14 +223,17 @@ lookupTopBndrRn_maybe rdr_name
 
 -----------------------------------------------
 lookupExactOcc :: Name -> RnM Name
+-- See Note [Looking up Exact RdrNames]
 lookupExactOcc name
-  | isExternalName name = return name
-  | otherwise           = do { env <- getGlobalRdrEnv
-    			     ; let gres = lookupGRE_Name env name
-                             ; case gres of
-                                 []    -> return name
-                                 [gre] -> return (gre_name gre)
-                                 _     -> pprPanic "lookupExactOcc" (ppr name $$ ppr gres) }
+  | isExternalName name 
+  = return name
+  | otherwise           
+  = do { env <- getGlobalRdrEnv
+       ; let gres = lookupGRE_Name env name
+       ; case gres of
+           []    -> return name
+           [gre] -> return (gre_name gre)
+           _     -> pprPanic "lookupExactOcc" (ppr name $$ ppr gres) }
 
 -----------------------------------------------
 lookupInstDeclBndr :: Name -> RdrName -> RnM Name
@@ -352,6 +355,39 @@ newIPNameRn :: IPName RdrName -> TcRnIf m n (IPName Name)
 newIPNameRn ip_rdr = newIPName (mapIPName rdrNameOcc ip_rdr)
 \end{code}
 
+Note [Looking up Exact RdrNames]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Exact RdrNames are generated by Template Haskell.  See Note [Binders
+in Template Haskell] in Convert.
+
+For data types and classes have Exact system Names in the binding
+positions for constructors, TyCons etc.  For example
+    [d| data T = MkT Int |]
+when we splice in and Convert to HsSyn RdrName, we'll get
+    data (Exact (system Name "T")) = (Exact (system Name "MkT")) ...
+
+But, constructors and the like need External Names, not System Names!
+So we do the following
+
+ * In RnEnv.newGlobalBinder we spot Exact RdrNames that wrap a
+   non-External Name, and make an External name for it. This is
+   the name that goes in the GlobalRdrEnv
+
+ * When looking up an occurrence of an Exact name, done in
+   RnEnv.lookupExactOcc, we find the Name with the right unique in the
+   GlobalRdrEnv, and use the on from the envt -- it will be an
+   External Name in the case of the data type/constructor above.
+
+ * Exact names are also use for purely local binders generated
+   by TH, such as    \x_33. x_33
+   Both binder and occurrence are Exact RdrNames.  The occurrence
+   gets looked up in the LocalRdrEnv by RnEnv.lookupOccRn, and 
+   misses, because lookupLocalRdrEnv always returns Nothing for
+   an Exact Name.  Now we fall through to lookupExactOcc, which
+   will find the Name is not in the GlobalRdrEnv, so we just use
+   the Exact supplied Name.
+
+
 Note [Usage for sub-bndrs]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 If you have this
@@ -567,6 +603,10 @@ lookupBindGroupOcc :: Maybe NameSet  -- See notes on the (Maybe NameSet)
 --
 -- See Note [Looking up signature names]
 lookupBindGroupOcc mb_bound_names what rdr_name
+  | Just n <- isExact_maybe rdr_name
+  = do { n' <- lookupExactOcc n
+       ; check_local_name n' }
+  | otherwise
   = do  { local_env <- getLocalRdrEnv
         ; case lookupLocalRdrEnv local_env rdr_name of {
             Just n  -> check_local_name n;
@@ -582,7 +622,7 @@ lookupBindGroupOcc mb_bound_names what rdr_name
             [] | null gres -> bale_out_with empty
                | otherwise -> bale_out_with import_msg
         }}}
-    where
+  where
       check_local_name name 	-- The name is in scope, and not imported
   	  = case mb_bound_names of
   		  Just bound_names | not (name `elemNameSet` bound_names)