- pb_field_iter_t iter;
-
- if (!pb_field_iter_begin(&iter, fields, dest_struct))
- return; /* Empty message type */
+ pb_size_t old_tag = *(pb_size_t*)iter->pSize; /* Previous which_ value */
+ pb_size_t new_tag = iter->pos->tag; /* New which_ value */
+
+ if (old_tag == 0)
+ return true; /* Ok, no old data in union */
+
+ if (old_tag == new_tag)
+ return true; /* Ok, old data is of same type => merge */
+
+ /* Release old data. The find can fail if the message struct contains
+ * invalid data. */
+ if (!pb_field_iter_find(iter, old_tag))
+ PB_RETURN_ERROR(stream, "invalid union tag");
+
+ pb_release_single_field(iter);
+
+ /* Restore iterator to where it should be.
+ * This shouldn't fail unless the pb_field_t structure is corrupted. */
+ if (!pb_field_iter_find(iter, new_tag))
+ PB_RETURN_ERROR(stream, "iterator error");