feat: dynamic $$slots

src/compiler/compile/Component.ts

@@ -220,7 +220,8 @@ export default class Component {
 			this.add_var(node, {
 				name,
 				injected: true,
-				referenced: true
+				referenced: true,
+				reassigned: name === '$$slots'
 			});
 		} else if (name[0] === '$') {
 			this.add_var(node, {
@@ -721,7 +722,8 @@ export default class Component {
 			} else if (is_reserved_keyword(name)) {
 				this.add_var(node, {
 					name,
-					injected: true
+					injected: true,
+					reassigned: name === '$$slots'
 				});
 			} else if (name[0] === '$') {
 				if (name === '$' || name[1] === '$') {

test/runtime/samples/$$slot-dynamic/A.svelte

@@ -0,0 +1,28 @@
+<script>
+	$: data = toString($$slots);
+	$: stringified = toString($$slots);
+
+	export function getData() {
+		return data;
+	}
+
+	function toString(data) {
+		const result = {};
+		const sortedKeys = Object.keys(data).sort();
+		sortedKeys.forEach(key => result[key] = data[key]);
+		return JSON.stringify(result);
+	}
+</script>
+
+<slot></slot>
+<slot name="a"></slot>
+
+$$slots: {toString($$slots)} {stringified}
+
+{#if $$slots.b}
+	<div>
+		<slot name="b"></slot>
+	</div>
+{:else}
+	Slot b is not available
+{/if}

test/runtime/samples/$$slot-dynamic/_config.js

@@ -0,0 +1,23 @@
+export default {
+	html: `
+		<span>bye</span><span>default</span>
+		<span>hello a</span>
+		$$slots: {"a":true,"default":true} {"a":true,"default":true}
+		Slot b is not available
+	`,
+
+	async test({ assert, component, target }) {
+		assert.equal(component.getData(), '{"a":true,"default":true}');
+		
+		component.show_b = true;
+		assert.htmlEqual(target.innerHTML, `
+			<span>bye</span><span>default</span>
+			<span>hello a</span>
+			$$slots: {"a":true,"b":true,"default":true} {"a":true,"b":true,"default":true}
+			<div><span>hello b</span></div>
+		`);
+		assert.equal(component.getData(), '{"a":true,"b":true,"default":true}');
+
+		component.show_default = false;
+	}
+};

test/runtime/samples/$$slot-dynamic/main.svelte

@@ -0,0 +1,30 @@
+<script>
+	import A from "./A.svelte";
+	let a;
+	export let show_default = true;
+	export let show_a = true;
+	export let show_b = false;
+
+	export function getData() {
+		return a.getData();
+	}
+</script>
+
+<A bind:this={a}>
+	{#if show_a}
+		<svelte:fragment slot="a">
+			<span>hello a</span>
+		</svelte:fragment>
+	{/if}
+	{#if show_default}
+		<svelte:fragment slot="default">
+			<span>bye</span>
+			<span>default</span>
+		</svelte:fragment>
+	{/if}
+	{#if show_b}
+		<svelte:fragment slot="b">
+			<span>hello b</span>
+		</svelte:fragment>
+	{/if}
+</A>

test/runtime/samples/component-conditional-slot-10-default/Foo.svelte

@@ -0,0 +1,3 @@
+<div>
+  <slot>default fallback</slot>
+</div>

test/runtime/samples/component-conditional-slot-10-default/_config.js

@@ -0,0 +1,13 @@
+export default {
+	html: `
+		<div></div>
+		<div>default fallback</div>
+	`,
+	test({ assert, component, target }) {
+		component.condition = true;
+		assert.htmlEqual(target.innerHTML, `
+			<div>hello #1</div>
+			<div>hello #2</div>
+		`);
+	}
+};

test/runtime/samples/component-conditional-slot-10-default/main.svelte

@@ -0,0 +1,16 @@
+<script>
+	import Foo from "./Foo.svelte";
+	export let condition = false;
+</script>
+
+<Foo>
+	{#if condition}
+		hello #1
+	{/if}
+</Foo>
+
+<Foo>
+	{#if condition}
+		<svelte:fragment slot="default">hello #2</svelte:fragment>
+	{/if}
+</Foo>