Add a test for an obvious-seeming (but not actually legal) kind of ca…

…st attempted in issue rust-lang#115, downgrade bug to an err in type.ml so you get a better message.

src/boot/me/type.ml

@@ -261,9 +261,7 @@ let check_stmt (cx:Semant.ctxt) : (fn_ctx -> Ast.stmt -> unit) =
           else
             LTYPE_poly ((Array.map (fun p -> p.Common.node) params), ty)
       | Ast.MOD_ITEM_type _ ->
-          Common.bug
-            ()
-            "internal_check_mod_item_decl: unexpected mod item type"
+          Common.err None "Type-item used in non-type context"
   in
 
   let rec internal_check_base_lval

src/test/compile-fail/constructor-as-cast.rs

@@ -0,0 +1,13 @@
+// error-pattern: non-type context
+type base =
+  obj {
+    fn foo();
+  };
+obj derived() {
+  fn foo() {}
+  fn bar() {}
+}
+fn main() {
+  let derived d = derived();
+  let base b = base(d);
+}