Add lint test for invalid map tiles.

OpenRA.Game/Map/Map.cs

@@ -206,6 +206,8 @@ namespace OpenRA
 		public readonly MiniYaml NotificationDefinitions;
 		public readonly MiniYaml TranslationDefinitions;
 
+		public readonly Dictionary<CPos, TerrainTile> ReplacedInvalidTerrainTiles = new Dictionary<CPos, TerrainTile>();
+
 		// Generated data
 		public readonly MapGrid Grid;
 		public IReadOnlyPackage Package { get; private set; }
@@ -435,6 +437,7 @@ namespace OpenRA
 			{
 				if (!tileset.TryGetTileInfo(Tiles[uv], out var info))
 				{
+					ReplacedInvalidTerrainTiles[uv.ToCPos(this)] = Tiles[uv];
 					Tiles[uv] = tileset.DefaultTerrainTile;
 					info = tileset.GetTileInfo(tileset.DefaultTerrainTile);
 				}

OpenRA.Game/Map/TileReference.cs

@@ -23,6 +23,8 @@ namespace OpenRA
 		}
 
 		public override int GetHashCode() { return Type.GetHashCode() ^ Index.GetHashCode(); }
+
+		public override string ToString() { return Type + "," + Index; }
 	}
 
 	public struct ResourceTile

OpenRA.Mods.Common/Lint/CheckMapTiles.cs

@@ -0,0 +1,25 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2020 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System;
+using System.Linq;
+
+namespace OpenRA.Mods.Common.Lint
+{
+	public class CheckMapTiles : ILintMapPass
+	{
+		public void Run(Action<string> emitError, Action<string> emitWarning, ModData modData, Map map)
+		{
+			foreach (var kv in map.ReplacedInvalidTerrainTiles)
+				emitError("Cell {0} references invalid terrain tile {1}.".F(kv.Key, kv.Value));
+		}
+	}
+}