Merge pull request #2326 from sveltejs/gh-2325

skip array literal length optimization in presence of spread

src/compile/render-dom/wrappers/EachBlock.ts

@@ -92,9 +92,11 @@ export default class EachBlockWrapper extends Wrapper {
 
 		this.index_name = this.node.index || renderer.component.get_unique_name(`${this.node.context}_index`);
 
-		const fixed_length = node.expression.node.type === 'ArrayExpression'
-			? node.expression.node.elements.length
-			: null;
+		const fixed_length =
+			node.expression.node.type === 'ArrayExpression' &&
+			node.expression.node.elements.every(element => element.type !== 'SpreadElement')
+				? node.expression.node.elements.length
+				: null;
 
 		// hack the sourcemap, so that if data is missing the bug
 		// is easy to find
@@ -508,4 +510,4 @@ export default class EachBlockWrapper extends Wrapper {
 
 		block.builders.destroy.add_block(`@destroy_each(${iterations}, detaching);`);
 	}
-}
+}

test/runtime/samples/array-literal-spread-deopt/_config.js

@@ -0,0 +1,3 @@
+export default {
+	html: `<div>a</div> <div>b</div> <div>c</div>`
+};

test/runtime/samples/array-literal-spread-deopt/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	const foo = ['a', 'b', 'c'];
+</script>
+
+{#each [...foo] as item}
+	<div>{item}</div>
+{/each}