- // If the moving piece is a king, check whether the destination
- // square is attacked by the opponent. Castling moves are checked
- // for legality during move generation.
+ // Castling moves generation does not check if the castling path is clear of
+ // enemy attacks, it is delayed at a later time: now!
+ if (type_of(m) == CASTLING)
+ {
+ // After castling, the rook and king final positions are the same in
+ // Chess960 as they would be in standard chess.
+ to = relative_square(us, to > from ? SQ_G1 : SQ_C1);
+ Direction step = to > from ? WEST : EAST;
+
+ for (Square s = to; s != from; s += step)
+ if (attackers_to(s) & pieces(~us))
+ return false;
+
+ // In case of Chess960, verify that when moving the castling rook we do
+ // not discover some hidden checker.
+ // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
+ return !chess960
+ || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
+ }
+
+ // If the moving piece is a king, check whether the destination square is
+ // attacked by the opponent.