fix: account for shadowing children slot during migration (#14224)

Fixes #14171

.changeset/lazy-chefs-float.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: account for shadowing children slot during migration

packages/svelte/src/compiler/migrate/index.js

@@ -1300,6 +1300,21 @@ const template = {
 			existing_prop.needs_refine_type = false;
 		}
 
+		if (
+			slot_name === 'default' &&
+			path.some(
+				(parent) =>
+					(parent.type === 'SvelteComponent' ||
+						parent.type === 'Component' ||
+						parent.type === 'RegularElement' ||
+						parent.type === 'SvelteElement' ||
+						parent.type === 'SvelteFragment') &&
+					parent.attributes.some((attr) => (attr.type = 'LetDirective'))
+			)
+		) {
+			aliased_slot_name = `${name}_render`;
+			state.derived_conflicting_slots.set(aliased_slot_name, name);
+		}
 		name = aliased_slot_name ?? name;
 
 		if (node.fragment.nodes.length > 0) {

packages/svelte/tests/migrate/samples/shadowed-forwarded-slot/input.svelte

@@ -13,3 +13,13 @@
 		</MyComponent>
 	</div>
 </MyInput>
+
+<MyInput let:args>
+	<slot/>
+</MyInput>
+
+<MyInput>
+	<div let:args>
+		<slot/>
+	</div>
+</MyInput>

packages/svelte/tests/migrate/samples/shadowed-forwarded-slot/output.svelte

@@ -2,11 +2,13 @@
 	/**
 	 * @typedef {Object} Props
 	 * @property {import('svelte').Snippet} [label]
+	 * @property {import('svelte').Snippet} [children]
 	 */
 
 	/** @type {Props} */
-	let { label } = $props();
+	let { label, children } = $props();
 	const label_render = $derived(label);
+	const children_render = $derived(children);
 
 </script>
 
@@ -31,3 +33,17 @@
 		</div>
 	{/snippet}
 </MyInput>
+
+<MyInput >
+	{#snippet children({ args })}
+		{@render children_render?.()}
+	{/snippet}
+</MyInput>
+
+<MyInput>
+	<div >
+		{#snippet children({ args })}
+				{@render children_render?.()}
+					{/snippet}
+		</div>
+</MyInput>