The ‘castling out of check’ case suggests to me another definition again: if we consider the restriction on castling out of check to be an extension of the restriction on moving through check — in effect that the K can be captured en‐passant on its starting square, upon having decided to move — then check for castling purposes would always be calculated with an effetive K move for the joker. Iow the move of the J, and whether it gives check, is defined at ‘touch move’ time.
For the case of determining checkmate or stalemate, that would mean that the J would give check with the intersection of the moves of all pieces able to move pseudo‐legally. As such KJK would still lead to checkmate, but with most other combinations of material the J would not give passive check at all. But in e.g. KJKQ the J would give passive check as a K. With more complex pieces (esp. those that can be blocked — particularly the Vulture of the large Apothecary games) this would potentially be position dependent. (And I think it's a little subtler yet in the hypothetical case of a game with both a joker and two royals with disjoint movesets)
Ofc this is probably horribly inefficient to program, provided it's even deemed to make sense (I like it for castling‐out‐of‐check restrictions, but I'm ambivalent between it and Option 2 (by Greg's numbering) for check‐/stalemate — it avoids the surprising(?) behaviour of HG's example where the J checks as a piece that's no longer on‐board, in exchange for arguably slightly greater opaciity of definition), so take it or leave it :)
The ‘castling out of check’ case suggests to me another definition again: if we consider the restriction on castling out of check to be an extension of the restriction on moving through check — in effect that the K can be captured en‐passant on its starting square, upon having decided to move — then check for castling purposes would always be calculated with an effetive K move for the joker. Iow the move of the J, and whether it gives check, is defined at ‘touch move’ time.
For the case of determining checkmate or stalemate, that would mean that the J would give check with the intersection of the moves of all pieces able to move pseudo‐legally. As such KJK would still lead to checkmate, but with most other combinations of material the J would not give passive check at all. But in e.g. KJKQ the J would give passive check as a K. With more complex pieces (esp. those that can be blocked — particularly the Vulture of the large Apothecary games) this would potentially be position dependent. (And I think it's a little subtler yet in the hypothetical case of a game with both a joker and two royals with disjoint movesets)
Ofc this is probably horribly inefficient to program, provided it's even deemed to make sense (I like it for castling‐out‐of‐check restrictions, but I'm ambivalent between it and Option 2 (by Greg's numbering) for check‐/stalemate — it avoids the surprising(?) behaviour of HG's example where the J checks as a piece that's no longer on‐board, in exchange for arguably slightly greater opaciity of definition), so take it or leave it :)