From 54ef1c3c9ef6cecd968d5c1ed6ded3a1a201a870 Mon Sep 17 00:00:00 2001
From: "simonpj@microsoft.com" <unknown>
Date: Mon, 4 Aug 2008 14:15:03 +0000
Subject: [PATCH] Fix Trac #2433 (deriving Typeable)

---
 compiler/typecheck/TcDeriv.lhs |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs
index 7a2954a..e4d66a6 100644
--- a/compiler/typecheck/TcDeriv.lhs
+++ b/compiler/typecheck/TcDeriv.lhs
@@ -467,13 +467,17 @@ mkEqnHelp orig tvs cls cls_tys tc_app mtheta
 	          -- we want to check the instance tycon, not the family tycon
 
 	-- For standalone deriving (mtheta /= Nothing), 
-	-- check that all the data constructors are in scope
+	-- check that all the data constructors are in scope.
+	-- No need for this when deriving Typeable, becuase we don't need
+	-- the constructors for that.
 	-- By this time we know that the thing is algebraic
 	--	because we've called checkInstHead in derivingStandalone
 	; rdr_env <- getGlobalRdrEnv
 	; let hidden_data_cons = isAbstractTyCon rep_tc || any not_in_scope (tyConDataCons rep_tc)
 	      not_in_scope dc  = null (lookupGRE_Name rdr_env (dataConName dc))
-	; checkTc (isNothing mtheta || not hidden_data_cons) 
+	; checkTc (isNothing mtheta || 
+	  	   not hidden_data_cons ||
+		   className cls `elem` typeableClassNames) 
 		  (derivingHiddenErr tycon)
 
 	; mayDeriveDataTypeable <- doptM Opt_DeriveDataTypeable
-- 
1.7.10.4