Fix Trac #2433 (deriving Typeable)

diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs
index 7a2954a..e4d66a6 100644 (file)
--- 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), 
                  -- 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))
        -- 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
                  (derivingHiddenErr tycon)
 
        ; mayDeriveDataTypeable <- doptM Opt_DeriveDataTypeable