PeterAlan, on 2012-May-22, 08:05, said:
This is just saying that where there's UI you're not allowed to make the right choice, and that's not the law. The question is whether bidding on to 4♠ as opposed to passing 3♠ "could demonstrably have been suggested" by the failure to alert 3♣, and I can't see how it was.
South's thought process over 3S should be akin to:
"North believes I have a long club suit, at least within the confines of my opening 1D, and has therefore bid 3S. This presumably denies values in clubs, and is likely to show a minimum or a hand with no biddable 4-card red suit. If he had bid 3S after correctly alerting my splinter, that would DENY* game interest.
"My hand has improved substantially based on what partner most likely has, compared to what he has shown. This suggests bidding 4S: so I need to decide whether it's at all plausible for me to pass 3S. 3C is not forcing to game, because a GF splinter would go to 4C. Partner has shown (absent UI) something like QJTxxx/xx/Qxx/Kx, and I can expect game to be around 50% at best. That makes passing a logical alternative, even if partner's hand is a little better than this."
*: this assumes they don't play 3C as GF, and therefore 3S as a slam try.
My opinion (both as a player and as a director) is that 4S IS demonstrably suggested by the UI; it is less clear to me that passing is an LA as I had to try quite hard to construct a hand for North that didn't make game a decent contract.
Edit: Barmar's posited hand is indeed one that makes passing an LA. I had assumed North promises 8+ for his bid.