From 5d0d375db8e1cff6cb9f4d57d040fa8f796bf2dd Mon Sep 17 00:00:00 2001
From: Tilps <Tilps@users.noreply.github.com>
Date: Sat, 6 Feb 2021 14:12:44 +1100
Subject: [PATCH] Fix crash applying moves without a built tree. (#1518)

---
 src/mcts/node.cc | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/mcts/node.cc b/src/mcts/node.cc
index c1870843e1..4c9d0a853c 100644
--- a/src/mcts/node.cc
+++ b/src/mcts/node.cc
@@ -622,7 +622,12 @@ void NodeTree::MakeMove(Move move) {
       break;
     }
   }
-  if (new_head == nullptr) throw Exception("Invalid move!");
+  // If the node has edges populated and new_head is null then an illegal move
+  // was attempted.  This is not compatible with solid children implementation
+  // of ReleaseChildren, so abort here.
+  if (current_head_->HasChildren() && new_head == nullptr) {
+    throw Exception("Invalid move!");
+  }
   move = board.GetModernMove(move);
   current_head_->ReleaseChildrenExceptOne(new_head);
   new_head = current_head_->child_.get();