fix

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

@@ -167,7 +167,7 @@ export function VariableDeclaration(node, context) {
 						b.arrow(
 							[b.id('$$fn')],
 							b.sequence([
-								b.call(b.id('$$fn'), b.member(state_id, b.id('v'))),
+								b.chain_call(b.id('$$fn'), b.member(state_id, b.id('v'))),
 								b.call('$.set', state_id, b.member(state_id, b.id('v')))
 							])
 						)

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

@@ -98,7 +98,7 @@ export function VariableDeclaration(node, context) {
 				const invalidation_id = /** @type {Identifier} */ (pattern.elements[1]);
 				declarations.push(
 					b.declarator(state_id, value),
-					b.declarator(invalidation_id, b.arrow([b.id('$$fn')], b.call(b.id('$$fn'), state_id)))
+					b.declarator(invalidation_id, b.arrow([b.id('$$fn')], b.chain_call(b.id('$$fn'), state_id)))
 				);
 				continue;
 			}

packages/svelte/src/compiler/utils/builders.js

@@ -130,6 +130,17 @@ export function call(callee, ...args) {
 	};
 }
 
+/**
+ * @param {string | ESTree.Expression} callee
+ * @param {...(ESTree.Expression | ESTree.SpreadElement | false | undefined)} args
+ * @returns {ESTree.ChainExpression}
+ */
+export function chain_call(callee, ...args) {
+	const expression = /** @type {ESTree.SimpleCallExpression} */ (call(callee, ...args));
+	expression.optional = true;
+	return { type: 'ChainExpression', expression };
+}
+
 /**
  * @param {string | ESTree.Expression} callee
  * @param {...ESTree.Expression} args