move destructuring to the beginning of await block updates

src/compiler/compile/render_dom/wrappers/AwaitBlock.ts

@@ -60,7 +60,7 @@ class AwaitBlockBranch extends Wrapper {
 
 			this.block.chunks.declarations.push(b`(${node.pattern} = #ctx[${index}])`);
 			if (this.block.has_update_method) {
-				this.block.chunks.update.push(b`(${node.pattern} = #ctx[${index}])`);
+				this.block.chunks.update.unshift(b`(${node.pattern} = #ctx[${index}])`);
 			}
 		}
 	}

test/runtime/samples/await-then-destruct-if/_config.js

@@ -0,0 +1,15 @@
+export default {
+	async test({ assert, target, component }) {
+		await Promise.resolve();
+
+		component.fail = 'wrong';
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			correct
+			correct
+			`
+		);
+	}
+};

test/runtime/samples/await-then-destruct-if/main.svelte

@@ -0,0 +1,24 @@
+<script>
+	import { writable } from 'svelte/store';
+
+	export let fail = 'incorrect'
+
+	const object = writable(Promise.resolve({ result: true }))
+	const array = writable(Promise.resolve([true]))
+</script>
+
+{#await $object then { result }}
+	{#if result}
+		correct
+	{:else}
+		{fail}
+	{/if}
+{/await}
+
+{#await $array then [result]}
+	{#if result}
+		correct
+	{:else}
+		{fail}
+	{/if}
+{/await}