invalid_annotation_target

The annotation '{0}' can only be used on {1}.

The analyzer produces this diagnostic when an annotation is applied to a kind of declaration that it doesn't support.

The following code produces this diagnostic because the optionalTypeArgs annotation isn't defined to be valid for top-level variables:

import 'package:meta/meta.dart';

@optionalTypeArgs
int x = 0;

Remove the annotation from the declaration.