sveltejs · Rich-Harris · May 15, 2019 · May 13, 2019
diff --git a/src/compile/Component.ts b/src/compile/Component.ts
@@ -152,10 +152,14 @@ export default class Component {
 		this.namespace = namespaces[this.component_options.namespace] || this.component_options.namespace;
 
 		if (compile_options.customElement) {
-			this.tag = this.component_options.tag || compile_options.tag;
-			if (!this.tag) {
-				throw new Error(`Cannot compile to a custom element without specifying a tag name via options.tag or <svelte:options>`);
+			if (this.component_options.tag === undefined && compile_options.tag === undefined) {
+				const svelteOptions = ast.html.children.find(child => child.name === 'svelte:options');
+				this.warn(svelteOptions, {
+					code: 'custom-element-no-tag',
+					message: `No custom element 'tag' option was specified. To automatically register a custom element, specify a name with a hyphen in it, e.g. <svelte:options tag="my-thing"/>. To hide this warning, use <svelte:options tag={null}/>`
+				});
 			}
+			this.tag = this.component_options.tag || compile_options.tag;
 		} else {
 			this.tag = this.name;
 		}
@@ -1265,9 +1269,9 @@ function process_component_options(component: Component, nodes) {
 						const message = `'tag' must be a string literal`;
 						const tag = get_value(attribute, code, message);
 
-						if (typeof tag !== 'string') component.error(attribute, { code, message });
+						if (typeof tag !== 'string' && tag !== null) component.error(attribute, { code, message });
 
-						if (!/^[a-zA-Z][a-zA-Z0-9]*-[a-zA-Z0-9-]+$/.test(tag)) {
+						if (tag && !/^[a-zA-Z][a-zA-Z0-9]*-[a-zA-Z0-9-]+$/.test(tag)) {
 							component.error(attribute, {
 								code: `invalid-tag-property`,
 								message: `tag name must be two or more words joined by the '-' character`

diff --git a/src/compile/render-dom/index.ts b/src/compile/render-dom/index.ts
@@ -462,9 +462,13 @@ export default function dom(
 
 				${body.length > 0 && body.join('\n\n')}
 			}
-
-			customElements.define("${component.tag}", ${name});
 		`);
+
+		if (component.tag != null) {
+			builder.add_block(deindent`
+				customElements.define("${component.tag}", ${name});
+			`);
+		}
 	} else {
 		const superclass = options.dev ? 'SvelteComponentDev' : 'SvelteComponent';
 

diff --git a/test/custom-elements/index.js b/test/custom-elements/index.js
@@ -5,6 +5,7 @@ import { rollup } from 'rollup';
 import * as virtual from 'rollup-plugin-virtual';
 import * as puppeteer from 'puppeteer';
 import { addLineNumbers, loadConfig, loadSvelte } from "../helpers.js";
+import { deepEqual } from 'assert';
 
 const page = `
 <body>
@@ -59,9 +60,11 @@ describe('custom-elements', function() {
 		const skip = /\.skip$/.test(dir);
 		const internal = path.resolve('internal.mjs');
 		const index = path.resolve('index.mjs');
+		const warnings = [];
 
 		(solo ? it.only : skip ? it.skip : it)(dir, async () => {
 			const config = loadConfig(`./custom-elements/samples/${dir}/_config.js`);
+			const expected_warnings = config.warnings || [];
 
 			const bundle = await rollup({
 				input: `test/custom-elements/samples/${dir}/test.js`,
@@ -84,6 +87,8 @@ describe('custom-elements', function() {
 									dev: config.dev
 								});
 
+								compiled.warnings.forEach(w => warnings.push(w));
+
 								return compiled.js;
 							}
 						}
@@ -112,6 +117,16 @@ describe('custom-elements', function() {
 			} catch (err) {
 				console.log(addLineNumbers(code));
 				throw err;
+			} finally {
+				if (expected_warnings) {
+					deepEqual(warnings.map(w => ({
+						code: w.code,
+						message: w.message,
+						pos: w.pos,
+						start: w.start,
+						end: w.end
+					})), expected_warnings);
+				}
 			}
 		});
 	});

diff --git a/test/custom-elements/samples/no-tag-warning/_config.js b/test/custom-elements/samples/no-tag-warning/_config.js
@@ -0,0 +1,17 @@
+export default {
+	warnings: [{
+		code: "custom-element-no-tag",
+		message: "No custom element 'tag' option was specified. To automatically register a custom element, specify a name with a hyphen in it, e.g. <svelte:options tag=\"my-thing\"/>. To hide this warning, use <svelte:options tag={null}/>",
+		pos: 0,
+		start: {
+			character: 0,
+			column: 0,
+			line: 1
+		},
+		end: {
+			character: 18,
+			column: 18,
+			line: 1
+		}
+	}]
+};
diff --git a/test/custom-elements/samples/no-tag-warning/main.svelte b/test/custom-elements/samples/no-tag-warning/main.svelte
@@ -0,0 +1,7 @@
+<svelte:options />
+
+<script>
+	export let name;
+</script>
+
+<h1>Hello {name}!</h1>
diff --git a/test/custom-elements/samples/no-tag-warning/test.js b/test/custom-elements/samples/no-tag-warning/test.js
@@ -0,0 +1,12 @@
+import * as assert from 'assert';
+import CustomElement from './main.svelte';
+
+export default function (target) {
+	customElements.define('no-tag', CustomElement);
+	target.innerHTML = `<no-tag name="world"></no-tag>`;
+
+	const el = target.querySelector('no-tag');
+	const h1 = el.shadowRoot.querySelector('h1');
+
+	assert.equal(h1.textContent, 'Hello world!');
+}
diff --git a/test/custom-elements/samples/no-tag/_config.js b/test/custom-elements/samples/no-tag/_config.js
@@ -0,0 +1,3 @@
+export default {
+	warnings: []
+};
diff --git a/test/custom-elements/samples/no-tag/main.svelte b/test/custom-elements/samples/no-tag/main.svelte
@@ -0,0 +1,7 @@
+<svelte:options tag={null} />
+
+<script>
+	export let name;
+</script>
+
+<h1>Hello {name}!</h1>
diff --git a/test/custom-elements/samples/no-tag/test.js b/test/custom-elements/samples/no-tag/test.js
@@ -0,0 +1,12 @@
+import * as assert from 'assert';
+import CustomElement from './main.svelte';
+
+export default function (target) {
+	customElements.define('no-tag', CustomElement);
+	target.innerHTML = `<no-tag name="world"></no-tag>`;
+
+	const el = target.querySelector('no-tag');
+	const h1 = el.shadowRoot.querySelector('h1');
+
+	assert.equal(h1.textContent, 'Hello world!');
+}