Better error messages for deriving

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 342114b..708b8e4 100644 (file)
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -2359,14 +2359,18 @@ tcSimplifyDeriv orig tyvars theta
        ; wanteds <- newDictBndrsO orig (substTheta tenv theta)
        ; (irreds, _) <- topCheckLoop doc wanteds
 
        ; wanteds <- newDictBndrsO orig (substTheta tenv theta)
        ; (irreds, _) <- topCheckLoop doc wanteds
 

-       ; let (dicts, non_dicts) = partition isDict irreds
-                                       -- Exclude implication consraints
-       ; addNoInstanceErrs non_dicts   -- I'm not sure if these can really happen
+       -- Insist that the context of a derived instance declaration
+       -- consists of constraints of form (C a b), where a,b are
+       -- type variables
+       -- NB: the caller will further check the tv_dicts for
+       --     legal instance-declaration form
+       ; let (tv_dicts, non_tv_dicts) = partition isTyVarDict irreds
+       ; addNoInstanceErrs non_tv_dicts

 
        ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
 
        ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)

-             simpl_theta = substTheta rev_env (map dictPred dicts)
-               -- This reverse-mapping is a Royal Pain, 
-               -- but the result should mention TyVars not TcTyVars
+             simpl_theta = substTheta rev_env (map dictPred tv_dicts)
+               -- This reverse-mapping is a pain, but the result
+               -- should mention the original TyVars not TcTyVars

 
        ; return simpl_theta }
   where
 
        ; return simpl_theta }
   where