Do not return unproven mate scores from null move search

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 4592ac084ebc56d60a59b83415167bd2f75af146..67c98709521ae5d63707a56d39a2f63b2a010d41 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1378,13 +1378,17 @@ namespace {
 
         if (nullValue >= beta)
         {
+            // Do not return unproven mate scores
+            if (nullValue >= value_mate_in(PLY_MAX))
+                nullValue = beta;
+
             if (depth < 6 * OnePly)
-                return beta;
+                return nullValue;
 
             // Do zugzwang verification search
             Value v = search(pos, ss, beta, depth-5*OnePly, ply, false, threadID);
             if (v >= beta)
-                return beta;
+                return nullValue;
         } else {
             // The null move failed low, which means that we may be faced with
             // some kind of threat. If the previous move was reduced, check if