minor fixes

src/mlw/dexpr.ml

@@ -1527,6 +1527,7 @@ and lambda uloc env pvl mask dsp dvl de =
   let env, old = add_label env old_mark in
   let e = if pvl = [] then expr uloc env de else
     let ity = ity_of_dity (dity_of_dvty de.de_dvty) in
+    let mask = if env.ghs then MaskGhost else mask in
     let xs = create_xsymbol old_mark_id ~mask ity in
     let e = expr uloc (add_xsymbol env xs) de in
     if not (Sxs.mem xs e.e_effect.eff_raises) then e else

src/mlw/expr.ml

@@ -916,7 +916,8 @@ let e_case e bl =
     | (_,d)::_ -> d.e_ity
     | [] -> invalid_arg "Expr.e_case" in
   List.iter (fun (p,d) ->
-    if mask_spill e.e_mask p.pp_mask then
+    if not (ity_equal e.e_ity ity_unit) &&
+        mask_spill e.e_mask p.pp_mask then
       Loc.errorm "Non-ghost pattern in a ghost position";
     ity_equal_check d.e_ity ity;
     ity_equal_check e.e_ity p.pp_ity) bl;
@@ -969,7 +970,8 @@ let e_try e xl =
 
 let e_raise xs e ity =
   ity_equal_check e.e_ity xs.xs_ity;
-  let ghost = mask_spill e.e_mask xs.xs_mask in
+  let ghost = not (ity_equal e.e_ity ity_unit) &&
+                mask_spill e.e_mask xs.xs_mask in
   let eff = eff_ghostify ghost (eff_raise eff_empty xs) in
   let eff = try_effect [e] eff_union_seq e.e_effect eff in
   mk_expr (Eraise (xs,e)) ity MaskVisible eff

src/mlw/ity.ml

@@ -877,6 +877,7 @@ let freeze_xs xs s = ity_freeze s xs.xs_ity
 
 let create_xsymbol id ?(mask=MaskVisible) ity =
   mask_check (Invalid_argument "Ity.create_xsymbol") ity mask;
+  let mask = if ity_equal ity ity_unit then MaskVisible else mask in
   { xs_name = id_register id; xs_ity = ity; xs_mask = mask_reduce mask }
 
 module Exn = MakeMSH (struct