support const tag for if block

src/compiler/compile/compiler_errors.ts

@@ -260,7 +260,7 @@ export default {
 	},
 	invalid_const_placement: {
 		code: 'invalid-const-placement',
-		message: '{@const} must be the immediate child of {#each}, {:then}, {:catch}, <svelte:fragment> or <Component>'
+		message: '{@const} must be the immediate child of {#if}, {#else}, {#each}, {:then}, {:catch}, <svelte:fragment> or <Component>'
 	},
 	invalid_const_declaration: (name: string) => ({
 		code: 'invalid-const-declaration',

src/compiler/compile/nodes/ConstTag.ts

@@ -11,7 +11,7 @@ import is_reference, { NodeWithPropertyDefinition } from 'is-reference';
 import get_object from '../utils/get_object';
 import compiler_errors from '../compiler_errors';
 
-const allowed_parents = new Set(['EachBlock', 'CatchBlock', 'ThenBlock', 'InlineComponent', 'SlotTemplate']);
+const allowed_parents = new Set(['EachBlock', 'CatchBlock', 'ThenBlock', 'InlineComponent', 'SlotTemplate', 'IfBlock', 'ElseBlock']);
 
 export default class ConstTag extends Node {
 	type: 'ConstTag';

src/compiler/compile/nodes/ElseBlock.ts

@@ -1,16 +1,21 @@
-import map_children from './shared/map_children';
 import AbstractBlock from './shared/AbstractBlock';
 import Component from '../Component';
 import TemplateScope from './shared/TemplateScope';
 import { TemplateNode } from '../../interfaces';
 import Node from './shared/Node';
+import ConstTag from './ConstTag';
+import get_const_tags from './shared/get_const_tags';
 
 export default class ElseBlock extends AbstractBlock {
 	type: 'ElseBlock';
+	scope: TemplateScope;
+	const_tags: ConstTag[];
 
 	constructor(component: Component, parent: Node, scope: TemplateScope, info: TemplateNode) {
 		super(component, parent, scope, info);
-		this.children = map_children(component, this, scope, info.children);
+
+		this.scope = scope.child();
+		([this.const_tags, this.children] = get_const_tags(info.children, component, this, this));
 
 		this.warn_if_empty_block();
 	}

src/compiler/compile/nodes/IfBlock.ts

@@ -1,22 +1,26 @@
 import ElseBlock from './ElseBlock';
 import Expression from './shared/Expression';
-import map_children from './shared/map_children';
 import AbstractBlock from './shared/AbstractBlock';
 import Component from '../Component';
 import TemplateScope from './shared/TemplateScope';
 import { TemplateNode } from '../../interfaces';
 import Node from './shared/Node';
+import ConstTag from './ConstTag';
+import get_const_tags from './shared/get_const_tags';
 
 export default class IfBlock extends AbstractBlock {
 	type: 'IfBlock';
 	expression: Expression;
 	else: ElseBlock;
+	scope: TemplateScope;
+	const_tags: ConstTag[];
 
 	constructor(component: Component, parent: Node, scope: TemplateScope, info: TemplateNode) {
 		super(component, parent, scope, info);
+		this.scope = scope.child();
 
-		this.expression = new Expression(component, this, scope, info.expression);
+		this.expression = new Expression(component, this, this.scope, info.expression);
-		this.children = map_children(component, this, scope, info.children);
+		([this.const_tags, this.children] = get_const_tags(info.children, component, this, this));
 
 		this.else = info.else
 			? new ElseBlock(component, this, scope, info.else)

src/compiler/compile/nodes/interfaces.ts

@@ -72,6 +72,8 @@ export type INode = Action
 | Window;
 
 export type INodeAllowConstTag =
+| IfBlock
+| ElseBlock
 | EachBlock
 | CatchBlock
 | ThenBlock

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

@@ -11,6 +11,7 @@ import { walk } from 'estree-walker';
 import { is_head } from './shared/is_head';
 import { Identifier, Node } from 'estree';
 import { push_array } from '../../../utils/push_array';
+import { add_const_tags, add_const_tags_context } from './shared/add_const_tags';
 
 function is_else_if(node: ElseBlock) {
 	return (
@@ -25,8 +26,10 @@ class IfBlockBranch extends Wrapper {
 	condition?: any;
 	snippet?: Node;
 	is_dynamic: boolean;
+	node: IfBlock | ElseBlock;
 
 	var = null;
+	get_ctx_name: Node | undefined;
 
 	constructor(
 		renderer: Renderer,
@@ -63,6 +66,8 @@ class IfBlockBranch extends Wrapper {
 			}
 		}
 
+		add_const_tags_context(renderer, this.node.const_tags);
+
 		this.block = block.child({
 			comment: create_debugging_comment(node, parent.renderer.component),
 			name: parent.renderer.component.get_unique_name(
@@ -74,6 +79,10 @@ class IfBlockBranch extends Wrapper {
 		this.fragment = new FragmentWrapper(renderer, this.block, node.children, parent, strip_whitespace, next_sibling);
 
 		this.is_dynamic = this.block.dependencies.size > 0;
+
+		if (node.const_tags.length > 0) {
+			this.get_ctx_name = parent.renderer.component.get_unique_name(is_else ? 'get_else_ctx' : 'get_if_ctx');
+		}
 	}
 }
 
@@ -190,6 +199,18 @@ export default class IfBlockWrapper extends Wrapper {
 		const has_outros = this.branches[0].block.has_outro_method;
 		const has_transitions = has_intros || has_outros;
 
+		this.branches.forEach(branch => {
+			if (branch.get_ctx_name) {
+				this.renderer.blocks.push(b`
+				function ${branch.get_ctx_name}(#ctx) {
+					const child_ctx = #ctx.slice();
+					${add_const_tags(block, branch.node.const_tags, 'child_ctx')}
+					return child_ctx;
+				}
+				`);
+			}
+		});
+
 		const vars = { name, anchor, if_exists_condition, has_else, has_transitions };
 
 		const detaching = parent_node && !is_head(parent_node) ? null : 'detaching';
@@ -260,34 +281,66 @@ export default class IfBlockWrapper extends Wrapper {
 	) {
 		const select_block_type = this.renderer.component.get_unique_name('select_block_type');
 		const current_block_type = block.get_unique_name('current_block_type');
+		const need_select_block_ctx = this.branches.some(branch => branch.get_ctx_name);
+		const select_block_ctx = need_select_block_ctx ? block.get_unique_name('select_block_ctx') : null;
+		const if_ctx = select_block_ctx ? x`${select_block_ctx}(#ctx, ${current_block_type})` : x`#ctx`;
+
 		const get_block = has_else
-			? x`${current_block_type}(#ctx)`
+			? x`${current_block_type}(${if_ctx})`
-			: x`${current_block_type} && ${current_block_type}(#ctx)`;
+			: x`${current_block_type} && ${current_block_type}(${if_ctx})`;
 
 		if (this.needs_update) {
 			block.chunks.init.push(b`
 				function ${select_block_type}(#ctx, #dirty) {
 					${this.branches.map(({ dependencies, condition, snippet }) => {
-						return b`${snippet && dependencies.length > 0 ? b`if (${block.renderer.dirty(dependencies)}) ${condition} = null;` : null}`;
+				return b`${snippet && dependencies.length > 0 ? b`if (${block.renderer.dirty(dependencies)}) ${condition} = null;` : null}`;
-					})}
+			})}
 					${this.branches.map(({ condition, snippet, block }) => condition
-					? b`
+				? b`
 						${snippet && b`if (${condition} == null) ${condition} = !!${snippet}`}
 						if (${condition}) return ${block.name};`
-					: b`return ${block.name};`
+				: b`return ${block.name};`
-					)}
+			)}
 				}
 			`);
 		} else {
 			block.chunks.init.push(b`
 				function ${select_block_type}(#ctx, #dirty) {
 					${this.branches.map(({ condition, snippet, block }) => condition
-					? b`if (${snippet || condition}) return ${block.name};`
+				? b`if (${snippet || condition}) return ${block.name};`
-					: b`return ${block.name};`)}
+				: b`return ${block.name};`)}
 				}
 			`);
 		}
 
+		if (need_select_block_ctx) {
+			// if all branches needs create a context
+			if (this.branches.every(branch => branch.get_ctx_name)) {
+				block.chunks.init.push(b`
+					function ${select_block_ctx}(#ctx, #type) {
+						${this.branches.map(({ condition, get_ctx_name, block }) => {
+							return condition
+								? b`if (#type === ${block.name}) return ${get_ctx_name}(#ctx);`
+								: b`return ${get_ctx_name}(#ctx);`;
+						}).filter(Boolean)}
+					}
+				`);
+			} else {
+				// when not all branches need to create a new context,
+				// this code is simpler
+				block.chunks.init.push(b`
+					function ${select_block_ctx}(#ctx, #type) {
+						${this.branches.map(({ get_ctx_name, block }) => {
+							return get_ctx_name
+								? b`if (#type === ${block.name}) return ${get_ctx_name}(#ctx);`
+								: null;
+						}).filter(Boolean)}
+						return #ctx;
+					}
+				`);
+			}
+		}
+
 		block.chunks.init.push(b`
 			let ${current_block_type} = ${select_block_type}(#ctx, ${this.renderer.get_initial_dirty()});
 			let ${name} = ${get_block};
@@ -322,7 +375,7 @@ export default class IfBlockWrapper extends Wrapper {
 			if (dynamic) {
 				block.chunks.update.push(b`
 					if (${current_block_type} === (${current_block_type} = ${select_block_type}(#ctx, #dirty)) && ${name}) {
-						${name}.p(#ctx, #dirty);
+						${name}.p(${if_ctx}, #dirty);
 					} else {
 						${change_block}
 					}
@@ -336,9 +389,9 @@ export default class IfBlockWrapper extends Wrapper {
 			}
 		} else if (dynamic) {
 			if (if_exists_condition) {
-				block.chunks.update.push(b`if (${if_exists_condition}) ${name}.p(#ctx, #dirty);`);
+				block.chunks.update.push(b`if (${if_exists_condition}) ${name}.p(${if_ctx}, #dirty);`);
 			} else {
-				block.chunks.update.push(b`${name}.p(#ctx, #dirty);`);
+				block.chunks.update.push(b`${name}.p(${if_ctx}, #dirty);`);
 			}
 		}
 
@@ -370,6 +423,9 @@ export default class IfBlockWrapper extends Wrapper {
 		const previous_block_index = block.get_unique_name('previous_block_index');
 		const if_block_creators = block.get_unique_name('if_block_creators');
 		const if_blocks = block.get_unique_name('if_blocks');
+		const need_select_block_ctx = this.branches.some(branch => branch.get_ctx_name);
+		const select_block_ctx = need_select_block_ctx ? block.get_unique_name('select_block_ctx') : null;
+		const if_ctx = select_block_ctx ? x`${select_block_ctx}(#ctx, ${current_block_type_index})` : x`#ctx`;
 
 		const if_current_block_type_index = has_else
 			? nodes => nodes
@@ -389,35 +445,63 @@ export default class IfBlockWrapper extends Wrapper {
 				? b`
 					function ${select_block_type}(#ctx, #dirty) {
 						${this.branches.map(({ dependencies, condition, snippet }) => {
-							return b`${snippet && dependencies.length > 0 ? b`if (${block.renderer.dirty(dependencies)}) ${condition} = null;` : null}`;
+					return b`${snippet && dependencies.length > 0 ? b`if (${block.renderer.dirty(dependencies)}) ${condition} = null;` : null}`;
-						})}
+				})}
 						${this.branches.map(({ condition, snippet }, i) => condition
-						? b`
+					? b`
 						${snippet && b`if (${condition} == null) ${condition} = !!${snippet}`}
 						if (${condition}) return ${i};`
-						: b`return ${i};`)}
+					: b`return ${i};`)}
 						${!has_else && b`return -1;`}
 					}
 				`
 				: b`
 					function ${select_block_type}(#ctx, #dirty) {
 						${this.branches.map(({ condition, snippet }, i) => condition
-						? b`if (${snippet || condition}) return ${i};`
+					? b`if (${snippet || condition}) return ${i};`
-						: b`return ${i};`)}
+					: b`return ${i};`)}
 						${!has_else && b`return -1;`}
 					}
 				`}
 		`);
 
+		if (need_select_block_ctx) {
+			// if all branches needs create a context
+			if (this.branches.every(branch => branch.get_ctx_name)) {
+				block.chunks.init.push(b`
+					function ${select_block_ctx}(#ctx, #index) {
+						${this.branches.map(({ condition, get_ctx_name }, i) => {
+							return condition
+								? b`if (#index === ${i}) return ${get_ctx_name}(#ctx);`
+								: b`return ${get_ctx_name}(#ctx);`;
+						}).filter(Boolean)}
+					}
+				`);
+			} else {
+				// when not all branches need to create a new context,
+				// this code is simpler
+				block.chunks.init.push(b`
+					function ${select_block_ctx}(#ctx, #index) {
+						${this.branches.map(({ get_ctx_name }, i) => {
+							return get_ctx_name
+								? b`if (#index === ${i}) return ${get_ctx_name}(#ctx);`
+								: null;
+						}).filter(Boolean)}
+						return #ctx;
+					}
+				`);
+			}
+		}
+
 		if (has_else) {
 			block.chunks.init.push(b`
 				${current_block_type_index} = ${select_block_type}(#ctx, ${this.renderer.get_initial_dirty()});
-				${name} = ${if_blocks}[${current_block_type_index}] = ${if_block_creators}[${current_block_type_index}](#ctx);
+				${name} = ${if_blocks}[${current_block_type_index}] = ${if_block_creators}[${current_block_type_index}](${if_ctx});
 			`);
 		} else {
 			block.chunks.init.push(b`
 				if (~(${current_block_type_index} = ${select_block_type}(#ctx, ${this.renderer.get_initial_dirty()}))) {
-					${name} = ${if_blocks}[${current_block_type_index}] = ${if_block_creators}[${current_block_type_index}](#ctx);
+					${name} = ${if_blocks}[${current_block_type_index}] = ${if_block_creators}[${current_block_type_index}](${if_ctx});
 				}
 			`);
 		}
@@ -445,10 +529,10 @@ export default class IfBlockWrapper extends Wrapper {
 			const create_new_block = b`
 				${name} = ${if_blocks}[${current_block_type_index}];
 				if (!${name}) {
-					${name} = ${if_blocks}[${current_block_type_index}] = ${if_block_creators}[${current_block_type_index}](#ctx);
+					${name} = ${if_blocks}[${current_block_type_index}] = ${if_block_creators}[${current_block_type_index}](${if_ctx});
 					${name}.c();
 				} else {
-					${dynamic && b`${name}.p(#ctx, #dirty);`}
+					${dynamic && b`${name}.p(${if_ctx}, #dirty);`}
 				}
 				${has_transitions && b`@transition_in(${name}, 1);`}
 				${name}.m(${update_mount_node}, ${anchor});
@@ -480,7 +564,7 @@ export default class IfBlockWrapper extends Wrapper {
 			if (dynamic) {
 				block.chunks.update.push(b`
 					if (${current_block_type_index} === ${previous_block_index}) {
-						${if_current_block_type_index(b`${if_blocks}[${current_block_type_index}].p(#ctx, #dirty);`)}
+						${if_current_block_type_index(b`${if_blocks}[${current_block_type_index}].p(${if_ctx}, #dirty);`)}
 					} else {
 						${change_block}
 					}
@@ -494,9 +578,9 @@ export default class IfBlockWrapper extends Wrapper {
 			}
 		} else if (dynamic) {
 			if (if_exists_condition) {
-				block.chunks.update.push(b`if (${if_exists_condition}) ${name}.p(#ctx, #dirty);`);
+				block.chunks.update.push(b`if (${if_exists_condition}) ${name}.p(${if_ctx}, #dirty);`);
 			} else {
-				block.chunks.update.push(b`${name}.p(#ctx, #dirty);`);
+				block.chunks.update.push(b`${name}.p(${if_ctx}, #dirty);`);
 			}
 		}
 
@@ -514,11 +598,12 @@ export default class IfBlockWrapper extends Wrapper {
 		detaching
 	) {
 		const branch = this.branches[0];
+		const if_ctx = branch.get_ctx_name ? x`${branch.get_ctx_name}(#ctx)` : x`#ctx`;
 
 		if (branch.snippet) block.add_variable(branch.condition, branch.snippet);
 
 		block.chunks.init.push(b`
-			let ${name} = ${branch.condition} && ${branch.block.name}(#ctx);
+			let ${name} = ${branch.condition} && ${branch.block.name}(${if_ctx});
 		`);
 
 		const initial_mount_node = parent_node || '#target';
@@ -533,15 +618,14 @@ export default class IfBlockWrapper extends Wrapper {
 
 			const enter = b`
 				if (${name}) {
-					${dynamic && b`${name}.p(#ctx, #dirty);`}
+					${dynamic && b`${name}.p(${if_ctx}, #dirty);`}
-					${
+					${has_transitions &&
-						has_transitions &&
+				b`if (${block.renderer.dirty(branch.dependencies)}) {
-						b`if (${block.renderer.dirty(branch.dependencies)}) {
 							@transition_in(${name}, 1);
 						}`
-					}
+				}
 				} else {
-					${name} = ${branch.block.name}(#ctx);
+					${name} = ${branch.block.name}(${if_ctx});
 					${name}.c();
 					${has_transitions && b`@transition_in(${name}, 1);`}
 					${name}.m(${update_mount_node}, ${anchor});
@@ -578,7 +662,7 @@ export default class IfBlockWrapper extends Wrapper {
 			}
 		} else if (dynamic) {
 			block.chunks.update.push(b`
-				if (${branch.condition}) ${name}.p(#ctx, #dirty);
+				if (${branch.condition}) ${name}.p(${if_ctx}, #dirty);
 			`);
 		}
 

src/compiler/compile/render_ssr/handlers/IfBlock.ts

@@ -1,17 +1,21 @@
 import IfBlock from '../../nodes/IfBlock';
 import Renderer, { RenderOptions } from '../Renderer';
 import { x } from 'code-red';
+import { get_const_tags } from './shared/get_const_tags';
+import { Node } from 'estree';
 
-export default function(node: IfBlock, renderer: Renderer, options: RenderOptions) {
+export default function (node: IfBlock, renderer: Renderer, options: RenderOptions) {
 	const condition = node.expression.node;
 
 	renderer.push();
 	renderer.render(node.children, options);
-	const consequent = renderer.pop();
+	let consequent: Node = renderer.pop();
+	if (node.const_tags.length > 0) consequent = x`(() => { ${get_const_tags(node.const_tags)}; return ${consequent} })()`;
 
 	renderer.push();
 	if (node.else) renderer.render(node.else.children, options);
-	const alternate = renderer.pop();
+	let alternate: Node = renderer.pop();
+	if (node.else && node.else.const_tags.length > 0) alternate = x`(() => { ${get_const_tags(node.else.const_tags)}; return ${alternate} })()`;
 
 	renderer.add_expression(x`${condition} ? ${consequent} : ${alternate}`);
 }

test/runtime/samples/const-tag-if-else-if/_config.js

@@ -0,0 +1,56 @@
+export default {
+	html: `
+		<div>20 x 40</div>
+		<div>20 x 40</div>
+	`,
+	props: {
+		boxes: [{ width: 20, height: 40 }]
+	},
+	async test({ component, target, assert }) {
+		component.boxes = [{ width: 40, height: 70 }];
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<div>40 x 70</div>
+			<div>40 x 70</div>
+		`
+		);
+		
+		component.boxes = [];
+
+		assert.htmlEqual(target.innerHTML, '');
+
+		component.boxes = [
+			{ width: 20, height: 40 },
+			{ width: 30, height: 50 }
+		];
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+		<div>20 x 40</div>
+		<div>30 x 50</div>
+		<div>20 x 40</div>
+		<div>30 x 50</div>
+		`
+		);
+
+		component.boxes = [
+			{ width: 80, height: 70 },
+			{ width: 90, height: 60 }
+		];
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+		<div>80 x 70</div>
+		<div>90 x 60</div>
+		<div>80 x 70</div>
+		<div>90 x 60</div>
+		`
+		);
+
+		component.boxes = [];
+		assert.htmlEqual(target.innerHTML, '');
+	}
+};

test/runtime/samples/const-tag-if-else-if/main.svelte

@@ -0,0 +1,24 @@
+<script>
+	export let boxes;
+</script>
+
+{#if boxes.length > 1}
+	{@const box1 = boxes[0]}
+	{@const box2 = boxes[1]}
+	{@const { width, height } = box1}
+	<div>{width} x {height}</div>
+	<div>{box2.width} x {box2.height}</div>
+{:else if boxes.length > 0}
+	{@const box = boxes[0]}
+	{@const { width, height } = box}
+	<div>{width} x {height}</div>
+{/if}
+
+{#if boxes.length > 1}
+	<div>{boxes[0].width} x {boxes[0].height}</div>
+	<div>{boxes[1].width} x {boxes[1].height}</div>
+{:else if boxes.length > 0}
+	{@const box = boxes[0]}
+	{@const { width, height } = box}
+	<div>{width} x {height}</div>
+{/if}

test/runtime/samples/const-tag-if-else-outro/_config.js

@@ -0,0 +1,57 @@
+export default {
+	html: `
+		<div>20 x 40</div>
+		<div>20 x 40</div>
+	`,
+	props: {
+		boxes: [{ width: 20, height: 40 }]
+	},
+	async test({ component, target, assert, raf }) {
+		component.boxes = [{ width: 40, height: 70 }];
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<div>40 x 70</div>
+			<div>40 x 70</div>
+		`
+		);
+
+		component.boxes = [];
+
+		raf.tick(0);
+		assert.htmlEqual(target.innerHTML, '');
+
+		component.boxes = [
+			{ width: 20, height: 40 },
+			{ width: 30, height: 50 }
+		];
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+		<div>20 x 40</div>
+		<div>30 x 50</div>
+		<div>20 x 40</div>
+		<div>30 x 50</div>
+		`
+		);
+
+		component.boxes = [
+			{ width: 80, height: 70 },
+			{ width: 90, height: 60 }
+		];
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+		<div>80 x 70</div>
+		<div>90 x 60</div>
+		<div>80 x 70</div>
+		<div>90 x 60</div>
+		`
+		);
+
+		component.boxes = [];
+		assert.htmlEqual(target.innerHTML, '');
+	}
+};

test/runtime/samples/const-tag-if-else-outro/main.svelte

@@ -0,0 +1,29 @@
+<script>
+	export let boxes;
+	function fade() {
+		return {
+			duration: 0,
+		}
+	}
+</script>
+
+{#if boxes.length > 1}
+	{@const box1 = boxes[0]}
+	{@const box2 = boxes[1]}
+	{@const { width, height } = box1}
+	<div>{width} x {height}</div>
+	<div>{box2.width} x {box2.height}</div>
+{:else if boxes.length > 0}
+	{@const box = boxes[0]}
+	{@const { width, height } = box}
+	<div out:fade>{width} x {height}</div>
+{/if}
+
+{#if boxes.length > 1}
+	<div>{boxes[0].width} x {boxes[0].height}</div>
+	<div>{boxes[1].width} x {boxes[1].height}</div>
+{:else if boxes.length > 0}
+	{@const box = boxes[0]}
+	{@const { width, height } = box}
+	<div out:fade>{width} x {height}</div>
+{/if}

test/runtime/samples/const-tag-if-else/_config.js

@@ -0,0 +1,43 @@
+export default {
+	html: '<div>20 x 40</div>',
+	props: {
+		boxes: [{ width: 20, height: 40 }]
+	},
+	async test({ component, target, assert }) {
+		component.boxes = [{ width: 30, height: 60 }];
+		
+		assert.htmlEqual(target.innerHTML, `
+		<div>30 x 60</div>
+		`);
+		
+		component.boxes = [
+			{ width: 20, height: 40 },
+			{ width: 30, height: 50 }
+		];
+		
+		assert.htmlEqual(target.innerHTML, `
+		<div>20 x 40</div>
+		<div>30 x 50</div>
+		`);
+
+		component.boxes = [
+			{ width: 80, height: 70 },
+			{ width: 90, height: 60 }
+		];
+		
+		assert.htmlEqual(target.innerHTML, `
+		<div>80 x 70</div>
+		<div>90 x 60</div>
+		`);
+		
+		component.boxes = [
+			{ width: 20, height: 40 },
+			{ width: 30, height: 50 },
+			{ width: 30, height: 50 }
+		];
+		assert.htmlEqual(target.innerHTML, '<div>3</div>');
+		
+		component.boxes = [];
+		assert.htmlEqual(target.innerHTML, '<div>0</div>');
+	}
+};

test/runtime/samples/const-tag-if-else/main.svelte

@@ -0,0 +1,18 @@
+<script>
+	export let boxes;
+</script>
+
+{#if boxes.length === 2}
+	{@const box1 = boxes[0]}
+	{@const box2 = boxes[1]}
+	{@const { width, height } = box1}
+	<div>{width} x {height}</div>
+	<div>{box2.width} x {box2.height}</div>
+{:else if boxes.length === 1}
+	{@const box = boxes[0]}
+	{@const { width, height } = box}
+	<div>{width} x {height}</div>
+{:else}
+	{@const length = boxes.length}
+	<div>{length}</div>
+{/if}

test/runtime/samples/const-tag-if/_config.js

@@ -0,0 +1,28 @@
+export default {
+	html: '<div>10 x 34</div>',
+	props: {
+		boxes: [{ width: 10, height: 34 }]
+	},
+	async test({ component, target, assert }) {
+		component.boxes = [{ width: 20, height: 40 }];
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+		<div>20 x 40</div>
+		`
+		);
+
+		component.boxes = [];
+		assert.htmlEqual(target.innerHTML, '');
+
+		component.boxes = [{ width: 18, height: 48 }];
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+		<div>18 x 48</div>
+		`
+		);
+	}
+};

test/runtime/samples/const-tag-if/main.svelte

@@ -0,0 +1,9 @@
+<script>
+	export let boxes;
+</script>
+
+{#if boxes.length > 0}
+	{@const box = boxes[0]}
+	{@const { width, height } = box}
+	<div>{width} x {height}</div>
+{/if}

test/validator/samples/const-tag-placement-1/errors.json

@@ -1,7 +1,7 @@
 [
 	{
 		"code": "invalid-const-placement",
-		"message": "{@const} must be the immediate child of {#each}, {:then}, {:catch}, <svelte:fragment> or <Component>",
+		"message": "{@const} must be the immediate child of {#if}, {#else}, {#each}, {:then}, {:catch}, <svelte:fragment> or <Component>",
 		"start": { "line": 5, "column": 0, "character": 36 },
 		"end": { "line": 5, "column": 18, "character": 54 },
 		"pos": 36

test/validator/samples/const-tag-placement-2/errors.json

@@ -1,9 +1,9 @@
 [
 	{
 		"code": "invalid-const-placement",
-		"message": "{@const} must be the immediate child of {#each}, {:then}, {:catch}, <svelte:fragment> or <Component>",
+		"message": "{@const} must be the immediate child of {#if}, {#else}, {#each}, {:then}, {:catch}, <svelte:fragment> or <Component>",
-		"start": { "line": 6, "column": 2, "character": 46 },
+		"start": { "line": 7, "column": 4, "character": 63 },
-		"end": { "line": 6, "column": 20, "character": 64 },
+		"end": { "line": 7, "column": 18, "character": 77 },
-		"pos": 46
+		"pos": 63
 	}
 ]

test/validator/samples/const-tag-placement-2/input.svelte

@@ -2,6 +2,8 @@
   export let a;
 </script>
 
-{#if a}
+{#each a as i}
-  {@const b = a + 1}
+  <div>
-{/if}
+    {@const b = i}
+  </div>
+{/each}

test/validator/samples/const-tag-placement-3/errors.json

@@ -1,9 +0,0 @@
-[
-	{
-		"code": "invalid-const-placement",
-		"message": "{@const} must be the immediate child of {#each}, {:then}, {:catch}, <svelte:fragment> or <Component>",
-		"start": { "line": 7, "column": 4, "character": 63 },
-		"end": { "line": 7, "column": 18, "character": 77 },
-		"pos": 63
-	}
-]

test/validator/samples/const-tag-placement-3/input.svelte

@@ -1,9 +0,0 @@
-<script>
-  export let a;
-</script>
-
-{#each a as i}
-  <div>
-    {@const b = i}
-  </div>
-{/each}