Error if @notundefined is used on a non-abstract type

Author: cknitt

compiler/ml/typedecl.ml

@@ -49,6 +49,7 @@ type error =
   | Unbound_type_var_ext of type_expr * extension_constructor
   | Varying_anonymous
   | Val_in_structure
+  | InvalidAttribute of string
   | Bad_immediate_attribute
   | Bad_unboxed_attribute of string
   | Boxed_and_unboxed
@@ -295,6 +296,20 @@ let make_constructor env type_path type_params sargs sret_type =
 *)
 
 let transl_declaration ~type_record_as_object ~untagged_wfc env sdecl id =
+  (* Check for @notUndefined attribute *)
+  let has_not_undefined =
+    List.exists (fun ({txt}, _) -> txt = "notUndefined") sdecl.ptype_attributes
+  in
+  (if has_not_undefined then
+     match (sdecl.ptype_kind, sdecl.ptype_manifest) with
+     | Ptype_abstract, None -> ()
+     | _ ->
+       raise
+         (Error
+            ( sdecl.ptype_loc,
+              InvalidAttribute
+                "@notUndefined can only be used on abstract types" )));
+
   (* Bind type parameters *)
   reset_type_variables ();
   Ctype.begin_def ();
@@ -2090,6 +2105,7 @@ let report_error ppf = function
       "The field @{<info>%s@} is defined several times in this record. Fields \
        can only be added once to a record."
       s
+  | InvalidAttribute msg -> fprintf ppf "%s" msg
   | Duplicate_label (s, Some record_name) ->
     fprintf ppf
       "The field @{<info>%s@} is defined several times in the record \