title: function "this" set some type context + original "this" context
body: <p>How do I merge given context of "this" with original "this" context I have in a function. Example:</p>
  
  <pre><code>interface Repository {
      find(): void
      findOne(): void
  }
  
  interface CustomRepository {
      [key: string]: (this: Repository &amp; this) =&gt; any // note: I tried to add &amp; this but it doesn't work.
  }
  
  const repository: CustomRepository = {
      findNew() {
          this.findOne()
      },
      findOld() {
          // here I would like to access find(), findOne(), findNew() using this.
          // this.
      }
  }
  </code></pre>
  
  <p>If there is issue / feature request / discussion in TypeScript repository, link is appreciated.</p>