svelte/test/runtime/samples/dev-warning-custom-event-destroy-not-teardown/main.html

<button on:foo='foo()'>foo</button>

<script>
	export default {
		methods: {
			foo() {}
		},
		events: {
			foo(node, callback) {
				return {
					teardown() {}
				}
			}
		}
	};
</script>