WIP solution for #1340

src/compiler/compile/nodes/shared/Expression.ts

@@ -138,8 +138,8 @@ export default class Expression {
 					}
 
 					if (template_scope.is_let(name)) {
-						if (!function_expression) {
-							dependencies.add(name);
+						if (!function_expression) { // TODO should this be `!lazy` ?
+							contextual_dependencies.add(name);
 						}
 					} else if (template_scope.names.has(name)) {
 						expression.uses_context = true;

test/runtime/samples/component-slot-let-in-binding/Nested.svelte

@@ -0,0 +1,9 @@
+<script>
+	export let items;
+</script>
+
+<div>
+	{#each items as item, index}
+		<slot {index}/>
+	{/each}
+</div>

test/runtime/samples/component-slot-let-in-binding/_config.js

@@ -0,0 +1,26 @@
+export default {
+	html: `
+		<div>
+			<label>1: <input></label>
+			<label>2: <input></label>
+			<label>3: <input></label>
+		</div>
+	`,
+
+	ssrHtml: `
+		<div>
+			<label>1: <input value="a"></label>
+			<label>2: <input value="b"></label>
+			<label>3: <input value="c"></label>
+		</div>
+	`,
+
+	async test({ assert, component, target, window }) {
+		const inputs = target.querySelectorAll('input');
+
+		inputs[2].value = 'd';
+		await inputs[2].dispatchEvent(new window.Event('input'));
+
+		assert.deepEqual(component.letters, ['a', 'b', 'd']);
+	}
+};

test/runtime/samples/component-slot-let-in-binding/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	import Nested from './Nested.svelte';
+
+	export let letters = ['a', 'b', 'c'];
+</script>
+
+<Nested items={letters} let:index>
+	<label>
+		{index + 1}: <input bind:value={letters[index]}>
+	</label>
+</Nested>