fix: deduplicate children prop from default slot

.changeset/cold-cheetahs-judge.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: deduplicate children prop and default slot

packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js

@@ -660,6 +660,13 @@ function serialize_inline_component(node, component_name, context) {
 	 */
 	let slot_scope_applies_to_itself = false;
 
+	/**
+	 * Components may have a children prop and also have child nodes. In this case, we assume
+	 * that the child component isn't using render tags yet and pass the slot as $$slots.default.
+	 * We're not doing it for spread attributes, as this would result in too many false positives.
+	 */
+	let has_children_prop = false;
+
 	/**
 	 * @param {import('estree').Property} prop
 	 */
@@ -709,6 +716,10 @@ function serialize_inline_component(node, component_name, context) {
 				slot_scope_applies_to_itself = true;
 			}
 
+			if (attribute.name === 'children') {
+				has_children_prop = true;
+			}
+
 			const [, value] = serialize_attribute_value(attribute.value, context);
 
 			if (attribute.metadata.dynamic) {
@@ -825,10 +836,13 @@ function serialize_inline_component(node, component_name, context) {
 			b.block([...(slot_name === 'default' && !slot_scope_applies_to_itself ? lets : []), ...body])
 		);
 
-		if (slot_name === 'default') {
+		if (slot_name === 'default' && !has_children_prop) {
 			push_prop(
 				b.init('children', context.state.options.dev ? b.call('$.wrap_snippet', slot_fn) : slot_fn)
 			);
+			// We additionally add the default slot as a boolean, so that the slot render function on the other
+			// side knows it should get the content to render from $$props.children
+			serialized_slots.push(b.init(slot_name, b.true));
 		} else {
 			serialized_slots.push(b.init(slot_name, slot_fn));
 		}
@@ -3057,7 +3071,7 @@ export const template_visitors = {
 					);
 
 		const expression = is_default
-			? b.member(b.id('$$props'), b.id('children'))
+			? b.call('$.default_slot', b.id('$$props'))
 			: b.member(b.member(b.id('$$props'), b.id('$$slots')), name, true, true);
 
 		const slot = b.call('$.slot', context.state.node, expression, props_expression, fallback);

packages/svelte/src/compiler/phases/3-transform/server/transform-server.js

@@ -970,6 +970,13 @@ function serialize_inline_component(node, component_name, context) {
 	 */
 	let slot_scope_applies_to_itself = false;
 
+	/**
+	 * Components may have a children prop and also have child nodes. In this case, we assume
+	 * that the child component isn't using render tags yet and pass the slot as $$slots.default.
+	 * We're not doing it for spread attributes, as this would result in too many false positives.
+	 */
+	let has_children_prop = false;
+
 	/**
 	 * @param {import('estree').Property} prop
 	 */
@@ -998,6 +1005,10 @@ function serialize_inline_component(node, component_name, context) {
 				slot_scope_applies_to_itself = true;
 			}
 
+			if (attribute.name === 'children') {
+				has_children_prop = true;
+			}
+
 			const value = serialize_attribute_value(attribute.value, context, false, true);
 			push_prop(b.prop('init', b.key(attribute.name), value));
 		} else if (attribute.type === 'BindDirective' && attribute.name !== 'this') {
@@ -1072,14 +1083,17 @@ function serialize_inline_component(node, component_name, context) {
 			b.block([...(slot_name === 'default' && !slot_scope_applies_to_itself ? lets : []), ...body])
 		);
 
-		if (slot_name === 'default') {
+		if (slot_name === 'default' && !has_children_prop) {
 			push_prop(
 				b.prop(
 					'init',
 					b.id('children'),
 					context.state.options.dev ? b.call('$.add_snippet_symbol', slot_fn) : slot_fn
 				)
 			);
+			// We additionally add the default slot as a boolean, so that the slot render function on the other
+			// side knows it should get the content to render from $$props.children
+			serialized_slots.push(b.init('default', b.true));
 		} else {
 			const slot = b.prop('init', b.literal(slot_name), slot_fn);
 			serialized_slots.push(slot);
@@ -1736,7 +1750,7 @@ const template_visitors = {
 		const lets = [];
 
 		/** @type {import('estree').Expression} */
-		let expression = b.member_id('$$props.children');
+		let expression = b.call('$.default_slot', b.id('$$props'));
 
 		for (const attribute of node.attributes) {
 			if (attribute.type === 'SpreadAttribute') {

packages/svelte/src/internal/client/dom/elements/custom-element.js

@@ -109,8 +109,9 @@ if (typeof HTMLElement === 'function') {
 				const existing_slots = get_custom_elements_slots(this);
 				for (const name of this.$$s) {
 					if (name in existing_slots) {
-						if (name === 'default') {
+						if (name === 'default' && !this.$$d.children) {
 							this.$$d.children = create_slot(name);
+							$$slots.default = true;
 						} else {
 							$$slots[name] = create_slot(name);
 						}

packages/svelte/src/internal/client/dom/legacy/misc.js

@@ -66,3 +66,15 @@ export function update_legacy_props($$new_props) {
 		}
 	}
 }
+
+/**
+ * @param {Record<string, any>} $$props
+ */
+export function default_slot($$props) {
+	var children = $$props.$$slots?.default;
+	if (children === true) {
+		return $$props.children;
+	} else {
+		return children;
+	}
+}

packages/svelte/src/internal/client/index.js

@@ -72,7 +72,8 @@ export {
 	add_legacy_event_listener,
 	bubble_event,
 	reactive_import,
-	update_legacy_props
+	update_legacy_props,
+	default_slot
 } from './dom/legacy/misc.js';
 export {
 	append,
@@ -154,7 +155,6 @@ export {
 } from './dom/operations.js';
 export { noop } from '../shared/utils.js';
 export {
-	add_snippet_symbol,
 	validate_component,
 	validate_dynamic_element_tag,
 	validate_snippet,

packages/svelte/src/internal/server/index.js

@@ -276,8 +276,9 @@ export function spread_attributes(attrs, lowercase_attributes, is_html, class_ha
 	for (let i = 0; i < attrs.length; i++) {
 		const obj = attrs[i];
 		for (key in obj) {
-			// omit functions
-			if (typeof obj[key] !== 'function') {
+			// omit functions and internal svelte properties
+			const prefix = key[0] + key[1]; // this is faster than key.slice(0, 2)
+			if (typeof obj[key] !== 'function' && prefix !== '$$') {
 				merged_attrs[key] = obj[key];
 			}
 		}
@@ -630,3 +631,5 @@ export {
 } from '../shared/validate.js';
 
 export { escape_html as escape };
+
+export { default_slot } from '../client/dom/legacy/misc.js';

packages/svelte/tests/runtime-legacy/samples/slot-children-prop/A.svelte

@@ -0,0 +1,6 @@
+<script>
+  export let children;
+</script>
+
+{children}
+<slot />

packages/svelte/tests/runtime-legacy/samples/slot-children-prop/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: `foo bar foo`
+});

packages/svelte/tests/runtime-legacy/samples/slot-children-prop/main.svelte

@@ -0,0 +1,9 @@
+<script>
+	import A from "./A.svelte"; 
+</script>
+
+<A children="foo">
+	bar
+</A>
+
+<A children="foo" />

packages/svelte/tests/snapshot/samples/function-prop-no-getter/_expected/client/index.svelte.js

@@ -21,7 +21,8 @@ export default function Function_prop_no_getter($$anchor) {
 
 			$.template_effect(() => $.set_text(text, `clicks: ${$.stringify($.get(count))}`));
 			$.append($$anchor, text);
-		}
+		},
+		$$slots: { default: true }
 	});
 
 	$.append($$anchor, fragment);

packages/svelte/tests/snapshot/samples/function-prop-no-getter/_expected/server/index.svelte.js

@@ -19,7 +19,8 @@ export default function Function_prop_no_getter($$payload, $$props) {
 		onmouseenter: () => count = plusOne(count),
 		children: ($$payload, $$slotProps) => {
 			$$payload.out += `clicks: ${$.escape(count)}`;
-		}
+		},
+		$$slots: { default: true }
 	});
 
 	$$payload.out += `<!--]-->`;