Positions with multiple stepping checkers not valid (fixes #1192)

Author: niklasf

chess/__init__.py

@@ -3654,7 +3654,8 @@ def status(self) -> Status:
         if self.was_into_check():
             errors |= STATUS_OPPOSITE_CHECK
 
-        # More than the maximum number of possible checkers in the variant.
+        # More than the maximum number of possible checkers in the variant,
+        # or impossibly aligned checkers.
         checkers = self.checkers_mask()
         our_kings = self.kings & self.occupied_co[self.turn] & ~self._effective_promoted()
         if checkers:
@@ -3673,6 +3674,11 @@ def status(self) -> Status:
                 if popcount(checkers) > 2 or (popcount(checkers) == 2 and ray(lsb(checkers), msb(checkers)) & our_kings):
                     errors |= STATUS_IMPOSSIBLE_CHECK
 
+            # Multiple steppers.
+            steppers = self.pawns | self.knights | self.kings
+            if popcount(checkers & steppers) > 1:
+                errors |= STATUS_IMPOSSIBLE_CHECK
+
         return errors
 
     def _valid_ep_square(self) -> Optional[Square]:

test.py

@@ -1042,6 +1042,10 @@ def test_status(self):
         board = chess.Board("8/8/5k2/p1q5/PP1rp1P1/3P1N2/2RK1r2/5nN1 w - - 0 3")
         self.assertEqual(board.status(), chess.STATUS_VALID)
 
+        # Multiple stepping checkers.
+        board = chess.Board("8/8/N7/2k5/N7/8/8/3K4 b - - 0 1")
+        self.assertEqual(board.status(), chess.STATUS_IMPOSSIBLE_CHECK)
+
     def test_one_king_movegen(self):
         board = chess.Board.empty()
         board.set_piece_at(chess.A1, chess.Piece(chess.KING, chess.WHITE))
@@ -4566,6 +4570,11 @@ def test_atomic_validity(self):
         board = chess.variant.AtomicBoard("3N1NB1/2N1Q1N1/3RkR2/2NP1PN1/3NKN2/8/8/n7 w - - 0 1")
         self.assertEqual(board.status(), chess.STATUS_VALID)
 
+        # Multiple stepping checkers possible, because opponent king may have
+        # moved away.
+        board = chess.variant.AtomicBoard("8/8/N7/2k1K3/N7/8/8/8 b - - 0 1")
+        self.assertEqual(board.status(), chess.STATUS_VALID)
+
     def test_atomic960(self):
         pgn = io.StringIO(textwrap.dedent("""\
             [Variant "Atomic"]