That wasn't the greatest example as Go doesn't really have any way to represent it. If it did it might be something like this.

  type MyStruct struct {
    Name string
  }

  var-not-nil myStruct = &MyStruct{"Hello"} // this is a "not-nil" pointer variable

  myStruct = nil // compiler would catch this

The idea being to allow variables to hold and pass pointers to structs like now but ensure they are never nil.

Now add an error to foo and have x=0 as a valid return value, or even worse, having to deal differently with different errors.