Here is my take: I think this Idea reinvents a more complex version of the wheel, because the current visitor pattern syntax is actually more straightforward then whatever he came up with. Now of course, let me warrant my claim here:
1. unlike match expressions (
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2688r3.html), it dosent reduce visual clutter in anyway that one can describe, in fact, it adds more variety of tokens to a typical switch statement has, hence calling it a short hand would be an extreme mistake, even when you compare it to traditional visitor calls or something similar, the syntax is much more cluttered, which again is a big issue if you are trying to introduce syntactical sugar in the first place.
2.Implementing something of this sort would blur the lines between what is considered a language feature vs a library one, which I get is already a little common but this one would just do it for the sake of some extra "sugar".
3.language are not expected to have the same amount of abi stability as library ones are, which is a no brainer since functions have abi interface and language features dont since you cant call an if statement a function that you could import from some other file, unless its in a header file(ie, it isn't a binary).
personally, I have tried to introduce such ideas, I regret trying to waste people's time, I do believe that at some point in life, you look at a idea worst than yours and say "christ on a cross, how is this even worse than mine", this idea definitely hits the spot, despite the fact that the person proposing it looks like a genuinely experienced developed unlike the 18 year old me. Again, it shows that even experienced developers can make worse judgement than a random 18 year old kid, who takes 200 emails to explain his dumb proposal to some really cool people online.