only do dirty check for writable values - fixes #2052

src/compile/render-dom/index.ts

@@ -314,7 +314,13 @@ export default function dom(
 
 	if (has_definition) {
 		const reactive_declarations = component.reactive_declarations.map(d => {
-			const condition = Array.from(d.dependencies).map(n => `$$dirty.${n}`).join(' || ');
+			const condition = Array.from(d.dependencies)
+				.filter(n => {
+					const variable = component.var_lookup.get(n);
+					return variable && variable.writable;
+				})
+				.map(n => `$$dirty.${n}`).join(' || ');
+
 			const snippet = d.node.body.type === 'BlockStatement'
 				? `[✂${d.node.body.start}-${d.node.end}✂]`
 				: deindent`

test/js/samples/reactive-values-non-writable-dependencies/expected.js

@@ -0,0 +1,40 @@
+/* generated by Svelte vX.Y.Z */
+import { SvelteComponent as SvelteComponent_1, init, noop, safe_not_equal } from "svelte/internal";
+
+function create_fragment(ctx) {
+	return {
+		c: noop,
+		m: noop,
+		p: noop,
+		i: noop,
+		o: noop,
+		d: noop
+	};
+}
+
+let a = 1;
+
+let b = 2;
+
+function instance($$self, $$props, $$invalidate) {
+
+
+	let max;
+
+	$$self.$$.update = ($$dirty = { max: 1, Math: 1, a: 1, b: 1 }) => {
+		if ($$dirty.max || $$dirty.a || $$dirty.b) {
+			max = Math.max(a, b); $$invalidate('max', max);
+		}
+	};
+
+	return {};
+}
+
+class SvelteComponent extends SvelteComponent_1 {
+	constructor(options) {
+		super();
+		init(this, options, instance, create_fragment, safe_not_equal);
+	}
+}
+
+export default SvelteComponent;

test/js/samples/reactive-values-non-writable-dependencies/input.html

@@ -0,0 +1,7 @@
+<script>
+	let a = 1;
+	let b = 2;
+
+	let max;
+	$: max = Math.max(a, b);
+</script>