From b6e386ee08fcd396a967fa682d10e985e122eb7d Mon Sep 17 00:00:00 2001
From: mrkishi <mauriciokishi@gmail.com>
Date: Mon, 23 Feb 2026 22:42:31 -0300
Subject: [PATCH] fix: handle default parameters scope leaks

Use separate scopes for function declarations/expressions and function
bodies. This prevents variable declarations from leaking into default
parameter initialization expressions.

Closes #17785.
---
 .changeset/modern-terms-refuse.md             |  5 ++++
 packages/svelte/src/compiler/phases/scope.js  | 23 ++++++++-----------
 .../function-parameter-shadowing/_config.js   |  7 ++++++
 .../function-parameter-shadowing/main.svelte  | 12 ++++++++++
 4 files changed, 34 insertions(+), 13 deletions(-)
 create mode 100644 .changeset/modern-terms-refuse.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/function-parameter-shadowing/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/function-parameter-shadowing/main.svelte

diff --git a/.changeset/modern-terms-refuse.md b/.changeset/modern-terms-refuse.md
new file mode 100644
index 0000000000..2a48bce096
--- /dev/null
+++ b/.changeset/modern-terms-refuse.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle default parameters scope leaks
diff --git a/packages/svelte/src/compiler/phases/scope.js b/packages/svelte/src/compiler/phases/scope.js
index 1e714942b3..9d563375d7 100644
--- a/packages/svelte/src/compiler/phases/scope.js
+++ b/packages/svelte/src/compiler/phases/scope.js
@@ -1098,25 +1098,19 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 		},
 
 		FunctionExpression(node, { state, next }) {
-			const scope = state.scope.child();
+			const scope = state.scope.child(true);
 			scopes.set(node, scope);
 
-			if (node.id) {
-				scopes.set(node.id, state.scope); // so that declarations within with the same name are not confused with the function name
-				scope.declare(node.id, 'normal', 'function');
-			}
+			if (node.id) scope.declare(node.id, 'normal', 'function');
 
 			add_params(scope, node.params);
 			next({ scope });
 		},
 
 		FunctionDeclaration(node, { state, next }) {
-			if (node.id) {
-				scopes.set(node.id, state.scope); // so that declarations within with the same name are not confused with the function name
-				state.scope.declare(node.id, 'normal', 'function', node);
-			}
+			if (node.id) state.scope.declare(node.id, 'normal', 'function', node);
 
-			const scope = state.scope.child();
+			const scope = state.scope.child(true);
 			scopes.set(node, scope);
 
 			add_params(scope, node.params);
@@ -1124,7 +1118,7 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 		},
 
 		ArrowFunctionExpression(node, { state, next }) {
-			const scope = state.scope.child();
+			const scope = state.scope.child(true);
 			scopes.set(node, scope);
 
 			add_params(scope, node.params);
@@ -1142,8 +1136,11 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 				parent?.type === 'FunctionExpression' ||
 				parent?.type === 'ArrowFunctionExpression'
 			) {
-				// We already created a new scope for the function
-				context.next();
+				// The scopes created for the function nodes above handle the function identifier and
+				// parameters, but the block statement itself holds the non-porous function scope
+				const scope = context.state.scope.child();
+				scopes.set(node, scope);
+				context.next({ scope });
 			} else {
 				create_block_scope(node, context);
 			}
diff --git a/packages/svelte/tests/runtime-runes/samples/function-parameter-shadowing/_config.js b/packages/svelte/tests/runtime-runes/samples/function-parameter-shadowing/_config.js
new file mode 100644
index 0000000000..c35d4d7ff8
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/function-parameter-shadowing/_config.js
@@ -0,0 +1,7 @@
+import { test } from '../../test';
+
+export default test({
+	test({ assert, logs }) {
+		assert.deepEqual(logs, [42, 43]);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/function-parameter-shadowing/main.svelte b/packages/svelte/tests/runtime-runes/samples/function-parameter-shadowing/main.svelte
new file mode 100644
index 0000000000..2e7aef1742
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/function-parameter-shadowing/main.svelte
@@ -0,0 +1,12 @@
+<script>
+	let value = $state(42);
+
+	function shadow(output = value) {
+		const value = 1337;
+		return output;
+	}
+
+	console.log(shadow());
+	value += 1;
+	console.log(shadow());
+</script>