From 1dda7d6486decf558d2ef848af3c22ee945801e6 Mon Sep 17 00:00:00 2001
From: Tyler Trahan <tyler@tylertrahan.com>
Date: Sun, 31 Jan 2021 04:04:22 -0500
Subject: Fix: don't walk out of the map when trying to build tunnels (#8600)

---
 src/town_cmd.cpp | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/town_cmd.cpp b/src/town_cmd.cpp
index e87c4437f..1aadd5927 100644
--- a/src/town_cmd.cpp
+++ b/src/town_cmd.cpp
@@ -1256,6 +1256,7 @@ static bool GrowTownWithTunnel(const Town* t, const TileIndex tile, const DiagDi
 		/* Only tunnel under a mountain if the slope is continuous for at least 4 tiles. We want tunneling to be a last resort for large hills. */
 		TileIndex slope_tile = tile;
 		for (uint8 tiles = 0; tiles < 4; tiles++) {
+			if (!IsValidTile(slope_tile)) return false;
 			slope = GetTileSlope(slope_tile);
 			if (slope != InclinedSlope(tunnel_dir) && !IsSteepSlope(slope) && !IsSlopeWithOneCornerRaised(slope)) return false;
 			slope_tile += delta;
-- 
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