wrap fallback hydration code in conditional

src/generators/dom/visitors/Slot.ts

@@ -51,6 +51,7 @@ export default function visitSlot(
 	);
 
 	block.builders.create.pushCondition(`!${content_name}`);
+	block.builders.hydrate.pushCondition(`!${content_name}`);
 	block.builders.mount.pushCondition(`!${content_name}`);
 	block.builders.unmount.pushCondition(`!${content_name}`);
 	block.builders.destroy.pushCondition(`!${content_name}`);
@@ -60,6 +61,7 @@ export default function visitSlot(
 	});
 
 	block.builders.create.popCondition();
+	block.builders.hydrate.popCondition();
 	block.builders.mount.popCondition();
 	block.builders.unmount.popCondition();
 	block.builders.destroy.popCondition();

src/utils/CodeBuilder.ts

@@ -5,12 +5,17 @@ enum ChunkType {
 	Block
 }
 
+interface Condition {
+	condition: string;
+	used: boolean;
+}
+
 export default class CodeBuilder {
 	result: string;
 	first: ChunkType;
 	last: ChunkType;
 	lastCondition: string;
-	conditionStack: string[];
+	conditionStack: Condition[];
 	indent: string;
 
 	constructor(str = '') {
@@ -28,6 +33,8 @@ export default class CodeBuilder {
 	}
 
 	addConditional(condition: string, body: string) {
+		this.reifyConditions();
+
 		body = body.replace(/^/gm, `${this.indent}\t`);
 
 		if (condition === this.lastCondition) {
@@ -45,6 +52,8 @@ export default class CodeBuilder {
 	}
 
 	addLine(line: string) {
+		this.reifyConditions();
+
 		if (this.lastCondition) {
 			this.result += `\n${this.indent}}`;
 			this.lastCondition = null;
@@ -63,6 +72,8 @@ export default class CodeBuilder {
 	}
 
 	addLineAtStart(line: string) {
+		this.reifyConditions();
+
 		if (this.first === ChunkType.Block) {
 			this.result = `${line}\n\n${this.indent}${this.result}`;
 		} else if (this.first === ChunkType.Line) {
@@ -76,6 +87,8 @@ export default class CodeBuilder {
 	}
 
 	addBlock(block: string) {
+		this.reifyConditions();
+
 		if (this.indent) block = block.replace(/^/gm, `${this.indent}`);
 
 		if (this.lastCondition) {
@@ -94,6 +107,8 @@ export default class CodeBuilder {
 	}
 
 	addBlockAtStart(block: string) {
+		this.reifyConditions();
+
 		if (this.result) {
 			this.result = `${block}\n\n${this.indent}${this.result}`;
 		} else {
@@ -109,15 +124,37 @@ export default class CodeBuilder {
 	}
 
 	pushCondition(condition: string) {
-		this.conditionStack.push(condition);
-		this.addLine(`if (${condition}) {`);
-		this.indent = repeat('\t', this.conditionStack.length);
+		this.conditionStack.push({ condition, used: false });
 	}
 
 	popCondition() {
-		this.conditionStack.pop();
+		const { used } = this.conditionStack.pop();
+
 		this.indent = repeat('\t', this.conditionStack.length);
-		this.addLine('}');
+		if (used) this.addLine('}');
+	}
+
+	reifyConditions() {
+		for (let i = 0; i < this.conditionStack.length; i += 1) {
+			const condition = this.conditionStack[i];
+			if (!condition.used) {
+				const line = `if (${condition.condition}) {`;
+
+				if (this.last === ChunkType.Block) {
+					this.result += `\n\n${this.indent}${line}`;
+				} else if (this.last === ChunkType.Line) {
+					this.result += `\n${this.indent}${line}`;
+				} else {
+					this.result += line;
+				}
+
+				this.last = ChunkType.Line;
+				if (!this.first) this.first = ChunkType.Line;
+
+				this.indent = repeat('\t', this.conditionStack.length);
+				condition.used = true;
+			}
+		}
 	}
 
 	toString() {

test/helpers.js

@@ -120,12 +120,16 @@ function cleanChildren(node) {
 }
 
 export function normalizeHtml(window, html) {
-	const node = window.document.createElement('div');
-	node.innerHTML = html
-		.replace(/>[\s\r\n]+</g, '><')
-		.trim();
-	cleanChildren(node, '');
-	return node.innerHTML.replace(/<\/?noscript\/?>/g, '');
+	try {
+		const node = window.document.createElement('div');
+		node.innerHTML = html
+			.replace(/>[\s\r\n]+</g, '><')
+			.trim();
+		cleanChildren(node, '');
+		return node.innerHTML.replace(/<\/?noscript\/?>/g, '');
+	} catch (err) {
+		throw new Error(`Failed to normalize HTML:\n${html}`);
+	}
 }
 
 export function setupHtmlEqual() {

test/runtime/samples/component-slot-fallback/Nested.html

@@ -1,5 +1,5 @@
 <div>
-	<slot>default fallback content</slot>
-	<slot name='bar'>bar fallback content</slot>
-	<slot name='foo'>foo fallback content</slot>
+	<slot><p class='default'>default fallback content</p></slot>
+	<slot name='bar'><p class='default'>bar fallback content</p></slot>
+	<slot name='foo'><p class='default'>foo fallback content</p></slot>
 </div>

test/runtime/samples/component-slot-fallback/_config.js

@@ -1,9 +1,9 @@
 export default {
 	html: `
 		<div>
-			<slot>default fallback content</slot>
-			<slot name='bar'>bar fallback content</slot>
-			<slot name='foo'>foo fallback content</slot>
+			<slot><p>not fallback</p></slot>
+			<slot name='bar'><p class='default'>bar fallback content</p></slot>
+			<slot name='foo'><p class='default'>foo fallback content</p></slot>
 		</div>
 	`
 };

test/runtime/samples/component-slot-fallback/main.html

@@ -1,4 +1,6 @@
-<Nested ref:nested></Nested>
+<Nested ref:nested>
+	<p>not fallback</p>
+</Nested>
 
 <script>
 	import Nested from './Nested.html';