Don't invalidate `this`. Fixes #2184

src/compile/render-dom/index.ts

@@ -165,9 +165,14 @@ export default function dom(
 				}
 
 				if (node.type === 'AssignmentExpression') {
-					const names = node.left.type === 'MemberExpression'
-						? [getObject(node.left).name]
-						: extractNames(node.left);
+					let names = [];
+
+					if (node.left.type === 'MemberExpression') {
+						const left_object_name = getObject(node.left).name;
+						left_object_name && (names = [left_object_name]);
+					} else {
+						names = extractNames(node.left);
+					}
 
 					if (node.operator === '=' && nodes_match(node.left, node.right)) {
 						const dirty = names.filter(name => {

test/js/samples/dont-invalidate-this/expected.js

@@ -0,0 +1,42 @@
+/* generated by Svelte vX.Y.Z */
+import { SvelteComponent as SvelteComponent_1, addListener, createElement, detachNode, init, insert, noop, safe_not_equal } from "svelte/internal";
+
+function create_fragment(ctx) {
+	var input, dispose;
+
+	return {
+		c() {
+			input = createElement("input");
+			dispose = addListener(input, "input", make_uppercase);
+		},
+
+		m(target, anchor) {
+			insert(target, input, anchor);
+		},
+
+		p: noop,
+		i: noop,
+		o: noop,
+
+		d(detach) {
+			if (detach) {
+				detachNode(input);
+			}
+
+			dispose();
+		}
+	};
+}
+
+function make_uppercase() {
+	this.value = this.value.toUpperCase();
+}
+
+class SvelteComponent extends SvelteComponent_1 {
+	constructor(options) {
+		super();
+		init(this, options, null, create_fragment, safe_not_equal);
+	}
+}
+
+export default SvelteComponent;

test/js/samples/dont-invalidate-this/input.svelte

@@ -0,0 +1,6 @@
+<script>
+	function make_uppercase() {
+		this.value = this.value.toUpperCase();
+	}
+</script>
+<input on:input={make_uppercase}>