Angular 4/5 HttpClient:字符串类型的参数无法分配给"body"

Angular文档说:

The Angular docs say:

响应主体不会返回您可能需要的所有数据.有时 服务器返回特殊的标头或状态代码以指示某些 条件，并进行检查.为此，您可以 告诉HttpClient您想要完整的响应，而不仅仅是正文 带有观察选项:

The response body doesn't return all the data you may need. Sometimes servers return special headers or status codes to indicate certain conditions, and inspecting those can be necessary. To do this, you can tell HttpClient you want the full response instead of just the body with the observe option:

http
  .get<MyJsonData>('/data.json', {observe: 'response'})
  .subscribe(resp => {
    // Here, resp is of type HttpResponse<MyJsonData>.
    // You can inspect its headers:
    console.log(resp.headers.get('X-Custom-Header'));
    // And access the body directly, which is typed as MyJsonData as requested.
    console.log(resp.body.someField);
  });

但是当我尝试这样做时，我得到了一个编译时错误(尽管没有运行时错误，按预期方式工作):

But when I try that, I get a compilation time error (no runtime errors though, works as expected):

错误TS2345:类型为'{headers的参数:HttpHeaders;观察:字符串； }'不可分配给类型'{headers ?: HttpHeaders | {[header:string]:字符串|细绳[]; };观察?参数?:Ht ...'. 属性观察"的类型不兼容. 不能将类型"string"分配给类型" body"".

error TS2345: Argument of type '{ headers: HttpHeaders; observe: string; }' is not assignable to parameter of type '{ headers?: HttpHeaders | { [header: string]: string | string[]; }; observe?: "body"; params?: Ht...'. Types of property 'observe' are incompatible. Type 'string' is not assignable to type '"body"'.

为什么?我用"@angular/http": "^5.1.0"

这是我的代码版本:

  login(credentials: Credentials): Observable<any> {
    const options = {
      headers: new HttpHeaders({'Content-Type': 'application/json'}),
      observe: 'response'
    };
    return this.httpClient.post<any>(`${environment.USER_SERVICE_BASE_URL}`,
      {'username': credentials.username, 'password': credentials.password}, options)
      .map((res) => ...