为什么这个函数隐含了任何返回类型？

Question

--strict这个

const arr = [''];
arr.map(_ => Observable.of({ json: () => [''] }));

导致此错误 [ts] Function implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.

然而这没关系

const arr = [''];
const empty = () => [''];
arr.map(_ => Observable.of({ json: empty }));

为什么？