Improve typechecker error messages; see Trac #1221

diff --git a/compiler/typecheck/TcUnify.lhs b/compiler/typecheck/TcUnify.lhs
index 99cd7b9..8de9f6c 100644 (file)
--- a/compiler/typecheck/TcUnify.lhs
+++ b/compiler/typecheck/TcUnify.lhs
@@ -721,8 +721,12 @@ tc_sub1 sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
 -----------------------------------
 defer_to_boxy_matching sub_ctxt act_sty actual_ty exp_ib exp_sty expected_ty
   = do { addSubCtxt sub_ctxt act_sty exp_sty $
-         u_tys True False act_sty actual_ty exp_ib exp_sty expected_ty
+         u_tys outer False act_sty actual_ty exp_ib exp_sty expected_ty
        ; return idHsWrapper }
+  where
+    outer = case sub_ctxt of           -- Ugh
+               SubDone -> False
+               other   -> True
 
 -----------------------------------
 tc_sub_funs act_arg act_res exp_ib exp_arg exp_res