fix: handle shadowed function names correctly (#17753)

Fixes #17750 (though the change that causes the issue only surfaced this
more general bug)

We were not adding the correct scope to function ids. Instead it was
part of the function body/params scope, which leads to bugs when the
function name is shadowed within the function.

.changeset/sad-eggs-read.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle shadowed function names correctly

packages/svelte/src/compiler/phases/scope.js

@@ -1101,14 +1101,20 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 			const scope = state.scope.child();
 			scopes.set(node, scope);
 
-			if (node.id) scope.declare(node.id, 'normal', 'function');
+			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');
+			}
 
 			add_params(scope, node.params);
 			next({ scope });
 		},
 
 		FunctionDeclaration(node, { state, next }) {
-			if (node.id) state.scope.declare(node.id, 'normal', 'function', node);
+			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);
+			}
 
 			const scope = state.scope.child();
 			scopes.set(node, scope);

packages/svelte/tests/runtime-runes/samples/derived-name-shadowed/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: `42`
+});

packages/svelte/tests/runtime-runes/samples/derived-name-shadowed/main.svelte

@@ -0,0 +1,8 @@
+<script module>
+	function foo() {
+		const foo = $derived(42);
+		return () => foo;
+	}
+</script>
+
+{foo()()}