Tom7 points out that FIDE article 5.2.b reads:
The game is drawn when a position has arisen in which neither player can checkmate the opponent’s king with any series of legal moves. The game is said to end in a ‘dead position’. This immediately ends the game, provided that the move producing the position was legal.
As Tom7 explains, examine the following position. After white plays Qxd7+ it is immediately the case that no legal sequence of moves can lead to a checkmate. This is true before black recaptures the queen — and therefore, black cannot recapture the queen, as the game is already a 'dead position' by FIDE rule 5.2.b, and the game is already over. Specifically, there is no legal continuation of the game in which either player is mated (helpmated or otherwise), and therefore the game is already over after Qxd7+ (1/2-1/2).
[FEN "3k4/3q4/8/5Q2/8/2K5/8/8 w - - 0 1"]
This seems to be the pretty clear conclusion of 5.2.b, and I don't see how to escape this. Sure, in practice, players would probably "play" Kxd7 as a convention, even though the game is already over before the invalid recapture.
But what if this situation arose in tournament?
- White plays Qxd7+.
- White hits the clock — I don't believe this is even necessary, as rule 6.2.a explicitly cites 5.2.b as "completing" a move immediately, due to the game immediately being over, much as it isn't necessary to hit the clock after delivering checkmate.
- Black's flag falls.
The lay misinterpretation of 5.2.b would be that black's flag fell while white still had a queen, and therefore black loses. But a close reading of 5.2.b and 6.2.a shows that the game was already over the moment the queen was placed next to the king, and black had no obligation to do anything further — just sign the score sheets and get lunch.
Has this ever come up in a tournament? I realize this is rules-lawyering, but e.g. Firouzja lost to Carlsen due to a possible helpmate, so I assume FIDE has at least some appetite for taking their own rules seriously.
Note: Closely related questions have been asked on SE, e.g. here, linking to resources like: http://wismuth.com/chess/illegal-moves.html. My question differs in that I'm asking about an incredibly common position. This sort of endgame occurs all the time, and is technically already dead, while these other questions are asking about "obscure" dead positions with bizarre locked up pawns and bishops and the like — I'm specifically asking about this incredibly common case of forced recaptures like the above, that I think most folks don't realize are already dead.