[TeeWEE]

Note that while interface upgrades are very usefull to use, they are not something very new in the programming language world.

In java you could do:

  if (reader instanceOf BufferedReader) {
    // upgrade
    BufferedReader bReader = (BufferedReader) reader;
  }

This is actualy adviced NOT to do, since its really a runtime type check.

In Go its no different, as this article explains. However its still a usefull mechanism sometimes.