interface Page<T> {
  title: string;
  content: string;
  lang: T;
}

function getPage<T> (
  title: string,
  content: string,
  lang: T,
): Page<T> {
  return {
    title,
    content,
    lang,
  }
}

class AppPage<T> {
  constructor (
    private title: string,
    private content: string,
    private lang: T,
  ) {}

  public get page () {
    return getPage(
      this.title,
      this.content,
      this.lang,
    )
  }
}

enum lang {
  ru = 'ru',
  en = 'en',
}

const appPage = new AppPage(
  'App',
  'Hello, World!',
  lang.en as const,
)

appPage.page

// WebStrom 
AppPage<lang.en>.page(): Page<T>
  
// VSCode
AppPage<lang.en>.page: Page<lang.en>