handle dynamic slot content

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

@@ -132,6 +132,10 @@ export default class SlotWrapper extends Wrapper {
 			const ${slot_definition} = ${renderer.reference('#slots')}.${slot_name};
 			const ${slot} = @create_slot(${slot_definition}, #ctx, ${renderer.reference('$$scope')}, ${get_slot_context_fn});
 			${has_fallback ? b`const ${slot_or_fallback} = ${slot} || ${this.fallback.name}(#ctx);` : null}
+			${has_fallback && this.renderer.options.customElement && this.renderer.component.component_options.tag
+				// This ensures that fallback content is rendered into the <slot> element given by the custom element wrapper
+				? b`if (${slot_or_fallback}.$$c_e) { ${slot_or_fallback}.$$c_e = ${this.fallback.name}(#ctx); }`
+				: null}
 		`);
 
 		block.chunks.create.push(

src/runtime/internal/Component.ts

@@ -172,25 +172,36 @@ if (typeof HTMLElement === 'function') {
 		connectedCallback() {
 			this.$$connected = true;
 			if (!this.$$component) {
-				function create_slot(name: string) {
+				function create_slot(name: string, $$c_e = false) {
 					return () => {
 						let node: HTMLSlotElement;
-						return {
+						const obj = {
 							c: function create() {
 								node = document.createElement('slot');
 								if (name !== 'default') {
 									node.setAttribute('name', name);
 								}
+								if (typeof obj.$$c_e === 'object') {
+									(obj.$$c_e as any).c()
+								}
 							},
 							m: function mount(target: HTMLElement, anchor?: HTMLElement) {
 								insert(target, node, anchor);
+								if (typeof obj.$$c_e === 'object') {
+									(obj.$$c_e as any).m(node, anchor)
+								}
 							},
 							d: function destroy(detaching: boolean) {
 								if (detaching) {
-									detach(node);
+									if (typeof obj.$$c_e === 'object') {
+										(obj.$$c_e as any).d(detaching)
 									}
+									detach(node);
 								}
+							},
+							$$c_e
 						};
+						return obj;
 					};
 				}
 
@@ -202,6 +213,15 @@ if (typeof HTMLElement === 'function') {
 					}
 				}
 
+				if (!Object.keys($$slots).length && this.$$slots.length) {
+					// There are potentially slots, but we didn't find any. This could be due to Svelte adding the child nodes
+					// only after the component is created (in the mount phase). In this case, pass in placeholder slots.
+					// The drawback is that all $$slots properties will be `true`, even if the component doesn't use them.
+					for (const name of this.$$slots) {
+						$$slots[name] = [create_slot(name, true)];
+					}
+				}
+
 				for (const attribute of this.attributes) {
 					// this.$$data takes precedence over this.attributes
 					const name = this.$$get_prop_name(attribute.name);

test/custom-elements/samples/$$slot-dynamic-content/main.svelte

@@ -0,0 +1,10 @@
+<svelte:options tag={null} />
+
+<script>
+	import "./my-widget.svelte";
+	export let name;
+</script>
+
+<my-widget>
+	<p>default {name}</p>
+</my-widget>

test/custom-elements/samples/$$slot-dynamic-content/my-widget.svelte

@@ -0,0 +1,4 @@
+<svelte:options tag="my-widget" />
+
+<slot>fallback</slot>
+<slot name="named"><p>named fallback</p></slot>

test/custom-elements/samples/$$slot-dynamic-content/test.js

@@ -0,0 +1,19 @@
+import * as assert from 'assert';
+import Component from './main.svelte';
+
+export default function (target) {
+	const component = new Component({ target, props: { name: 'slot' } });
+
+	const ce = target.querySelector('my-widget');
+
+	assert.htmlEqual(ce.shadowRoot.innerHTML, `
+		<slot>fallback</slot>
+		<slot name=\"named\"><p>named fallback</p></slot>
+	`);
+
+	component.name = 'slot2';
+	assert.htmlEqual(ce.shadowRoot.innerHTML, `
+		<slot>fallback</slot>
+		<slot name=\"named\"><p>named fallback</p></slot>
+	`);
+}