Fixed wrong flipped logic

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

@@ -259,15 +259,15 @@ export function serialize_set_binding(node, context, fallback) {
 						node.type === 'ArrowFunctionExpression'
 				)
 			);
-		const closest_function_boundary_not_async =
-			closest_function_boundary === undefined || closest_function_boundary.async !== true;
+		const closest_function_boundary_is_async =
+			closest_function_boundary !== undefined && closest_function_boundary.async === true;
 
 		const rhs_expression = /** @type {import('estree').Expression} */ (visit(node.right));
 
 		const iife_is_async =
-			!closest_function_boundary_not_async ||
-			is_expression_async(rhs_expression) ||
-			assignments.some((assignment) => is_expression_async(assignment));
+			closest_function_boundary_is_async &&
+			(is_expression_async(rhs_expression) ||
+				assignments.some((assignment) => is_expression_async(assignment)));
 
 		const iife = b.arrow(
 			[],