Let me define the problem clearly so as to avoid confusion. I am trying to find a position where:
- The unique best move is a promotion move.
- Said move needs to be promotion to a bishop. Not a queen, not a knight, and not a rook.
- Where "unique best move" means either: a move that draws while all other moves lose, or, a move that wins while all other moves draw or lose.
I have been trying to come up with such a position for more than a week now without success. The following is my thought process.
AFAIK there are two main reasons for underpromoting:
- Underpromote to deliver a check/attack that a queen cannot fulfill
- Underpromote to avoid stalemate
The first scenario should be irrelevant because a queen has all the moves of a bishop.
For the second scenario, I thought about the fundamental cause of stalemate. Promoting to queen can draw, either due to its rook-move ability, or its bishop-move ability, or both abilities simultaneously.
|Reason for stalemate
|Promote to Bishop/Knight
|Promote to Rook/Knight
|Promote to Rook/Bishop/Knight
As you can see, there is no scenario where you have to get a bishop. Therefore I concluded that any singular reason alone cannot produce a position where promotion to bishop is the only move. In other words, there has to be other conditions in the position that prevent the choice of another piece, in particular that of the knight.
One idea that came to my mind is that you might need a bishop-move to prevent something, but this idea got shot down pretty quickly because if you need to prevent something, that means the opponent is threatening a move, which means there's no stalemate.
Currently, the only semi-plausible idea I have left is that promotion to knight leads to insufficient material, whereas promotion to bishop doesn't. However coming up with such a position is quite difficult as you can imagine (especially given the condition that it has to be the only move), so I have yet to think of one.
I have asked this question to a few IMs and GMs on their streams to no avail (understandably so, since they still have to stream), so I am turning to StackExchange community for ideas and answers.