Swift: make unknown entity type check more solid

Author: redsun82

swift/extractor/infra/SwiftDispatcher.h

@@ -80,10 +80,11 @@ class SwiftDispatcher {
   void emitUnknown(E* entity) {
     auto label = assignNewLabel(entity);
     using Trap = BindingTrapOf<E>;
-    static_assert(sizeof(Trap) == sizeof(label),
-                  "Binding traps of unknown entities must only have the `id` field (the class "
-                  "should be empty in schema.yml)");
-    emit(Trap{label});
+    Trap entry{label};
+    // check that Trap only has the `id` field by using structured binding on it
+    auto [id] = entry;
+    std::ignore = id;
+    emit(entry);
     emit(ElementIsUnknownTrap{label});
   }