Do not skip non-recapture ttMove when in check

The qsearch() MovePicker incorrectly skips a non-recapture ttMove
when in check (if depth <= DEPTH_QS_RECAPTURES). This is clearly not
intended and can cause qsearch() to return a mate score when there
is no mate. Introduced in cad300c and 6596f0e, as observed by
joergoster in #3171 and #3198.

This PR fixes the bug by not skipping the non-recapture ttMove when in check.

Passed non-regression STC:
https://tests.stockfishchess.org/tests/view/5f9867ea6a2c112b60691b10
LLR: 2.98 (-2.94,2.94) {-1.25,0.25}
Total: 27112 W: 2943 L: 2842 D: 21327
Ptnml(0-2): 127, 2170, 8878, 2237, 144

Passed non-regression LTC:
https://tests.stockfishchess.org/tests/view/5f9967326a2c112b60691bb0
LLR: 2.99 (-2.94,2.94) {-0.75,0.25}
Total: 18392 W: 807 L: 738 D: 16847
Ptnml(0-2): 9, 655, 7802, 718, 12

closes https://github.com/official-stockfish/Stockfish/pull/3199
closes https://github.com/official-stockfish/Stockfish/pull/3198

Bench: 3870606

diff --git a/src/movepick.cpp b/src/movepick.cpp
index 153d323e8be87e262be8c80cdaa06269640a8859..f5e023855561063a15ab79770f11b1ed9b6d5257 100644 (file)
--- a/src/movepick.cpp
+++ b/src/movepick.cpp
@@ -73,8 +73,9 @@ MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHist
   assert(d <= 0);
 
   stage = (pos.checkers() ? EVASION_TT : QSEARCH_TT) +
   assert(d <= 0);
 
   stage = (pos.checkers() ? EVASION_TT : QSEARCH_TT) +

-           !(ttm && (depth > DEPTH_QS_RECAPTURES || to_sq(ttm) == recaptureSquare)
-                 && pos.pseudo_legal(ttm));
+          !(   ttm
+            && (pos.checkers() || depth > DEPTH_QS_RECAPTURES || to_sq(ttm) == recaptureSquare)
+            && pos.pseudo_legal(ttm));

 }
 
 /// MovePicker constructor for ProbCut: we generate captures with SEE greater
 }
 
 /// MovePicker constructor for ProbCut: we generate captures with SEE greater