seeking advice: possible new @attribute to mark class' default property to avoid alias this ?

[Paul Backus]

On Sunday, 8 August 2021 at 00:52:43 UTC, someone wrote:
> Now that I am aware of Walter's stance on alias this:
>
> "alias this has turned out to be a mistake" @ 
> https://news.ycombinator.com/item?id=28029184
>
> ... would you, I mean the community, think is it a good idea to 
> file a DIP to eventually get a new attribute to unambiguously 
> label a class' default property ?

Can you please explain what a "default property" is? I have never 
encountered such a thing in any other programming language.