It is inconvenient to do pattern matching on "this"

[rrousselGit]

One regular scenario when using extensions is to do pattern matching on this.

For example:

extension on num {
  void example() {
    if (this is int) doSomethingWithInts(this);
    else doSomethingWithDoubles(this);
  }
}

The problem is, this is not promoted after doing type-checks on it. This forces us to either use casts or dereference this.

For example a common pattern I've used is:

final that = this;
if (that is int)  doSomethingWithInts(that);

But this feels redundant. Would it be possible to promote this when doing type checks on it?

[halildurmus]

Duplicate of #1397

Reading the comments, it looks like this was supposed to be part of the promotion of the private final fields feature (#2020).

cc @stereotype441

[rrousselGit]

Closing since this is a dupp. Sorry I missed it