fixup ty collision detection

Author: gares

src/compiler/compiler.ml

@@ -1435,9 +1435,10 @@ end = struct
 
     let merge k a b =
       match a, b with
-      | _, Some (_,loc) -> Some loc
-      | Some _, b -> a
-      | _ -> anomaly ("ty_names collision: " ^ F.show k) in
+      | None, Some (_,loc) -> Some loc
+      | Some _, None -> a
+      | Some otherloc, Some (_,loc) -> error ~loc ("Duplicate type declaration: " ^ F.show k ^ ". Previous declaration: " ^ Loc.show otherloc)
+      | _ -> assert false in
     let ty_names = F.Map.merge merge ty_names kinds in
     let all_ty_names = F.Map.merge merge all_ty_names kinds in
     let types = Type_checker.check_types ~type_abbrevs:all_type_abbrevs ~kinds:all_kinds types in