tests pass

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

@@ -204,6 +204,9 @@ export default class EachBlockWrapper extends Wrapper {
 		const snippet = this.node.expression.manipulate(block);
 
 		block.chunks.init.push(b`let ${this.vars.each_block_value} = ${snippet};`);
+		if (this.renderer.options.dev) {
+			block.chunks.init.push(b`@validate_each_argument(${this.vars.each_block_value});`);
+		}
 
 		// TODO which is better — Object.create(array) or array.slice()?
 		renderer.blocks.push(b`
@@ -374,9 +377,12 @@ export default class EachBlockWrapper extends Wrapper {
 		block.chunks.init.push(b`
 			const ${get_key} = #ctx => ${this.node.key.manipulate(block)};
 
-			${this.renderer.options.dev && b`@validate_each_keys(#ctx, ${this.vars.each_block_value}, ${this.vars.get_each_context}, ${get_key});`}
+			${this.renderer.options.dev &&
+				b`@validate_each_keys(#ctx, ${this.vars.each_block_value}, ${this.vars.get_each_context}, ${get_key});`}
 			for (let #i = 0; #i < ${data_length}; #i += 1) {
-				let child_ctx = ${this.vars.get_each_context}(#ctx, ${this.vars.each_block_value}, #i);
+				let child_ctx = ${this.vars.get_each_context}(#ctx, ${
+			this.vars.each_block_value
+		}, #i);
 				let key = ${get_key}(child_ctx);
 				${lookup}.set(key, ${iterations}[#i] = ${create_each_block}(key, child_ctx));
 			}

src/runtime/internal/dev.ts

@@ -79,6 +79,17 @@ export function set_data_dev(text, data) {
 	text.data = data;
 }
 
+export function validate_each_argument(arg) {
+	if (arg instanceof Set || arg instanceof Map) {
+		throw new Error(`Svelte does not allow Sets or Maps in an {#each} block. Use [...arg.values()] instead.`);
+	} 
+	if (arg.length === undefined) {
+		throw new Error(
+			`Svelte needs an array-like value for the {#each} block. You can spread your iterable into an array instead, e.g. [...iterable]`
+		);
+	}
+}
+
 
 type Props = Record<string, any>;
 export interface SvelteComponentDev {

test/runtime/samples/dev-warning-each-block-no-sets-maps/_config.js

@@ -0,0 +1,6 @@
+export default {
+	compileOptions: {
+		dev: true
+	},
+	error: `Svelte does not allow Sets or Maps in an {#each} block. Use [...arg.values()] instead.`
+};

test/runtime/samples/dev-warning-each-block-no-sets-maps/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	const foo = new Set([1,2,3])
+</script>
+
+{#each foo as item}
+	<div>{item}</div>
+{/each}

test/runtime/samples/dev-warning-each-block-require-arraylike/_config.js

@@ -0,0 +1,6 @@
+export default {
+	compileOptions: {
+		dev: true
+	},
+	error: `Svelte needs an array-like value for the {#each} block. You can spread your iterable into an array instead, e.g. [...iterable]`
+};

test/runtime/samples/dev-warning-each-block-require-arraylike/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	const foo = new Set([1,2,3])
+</script>
+
+{#each foo.values() as item}
+	<div>{item}</div>
+{/each}