tests: add test that exposes sveltejs#3634

test/runtime/samples/reactive-compound-operator/_config.js

@@ -0,0 +1,27 @@
+export default {
+	html: `
+		<button>+1</button>
+		<p>count:0</p>
+	`,
+
+	async test({ assert, component, target, window }) {
+		const click = new window.MouseEvent('click');
+		const button = target.querySelector('button');
+
+		await button.dispatchEvent(click);
+
+		assert.equal(component.x, 2);
+		assert.htmlEqual(target.innerHTML, `
+			<button>+1</button>
+			<p>count:2</p>
+		`);
+
+		await button.dispatchEvent(click);
+
+		assert.equal(component.x, 6);
+		assert.htmlEqual(target.innerHTML, `
+			<button>+1</button>
+			<p>count:6</p>
+		`);
+	}
+};

test/runtime/samples/reactive-compound-operator/main.svelte

@@ -0,0 +1,8 @@
+<script>
+	export let x = 0;
+
+    $: x *= 2;
+</script>
+
+<button on:click='{() => x += 1}'>+1</button>
+<p>count:{x}</p>