Fix handling of custom bridge heads on steep slopes.

src/road_cmd.cpp

@@ -731,7 +731,11 @@ CommandCost CmdBuildRoad(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32
 				if ((pieces & ~axial_pieces) && !_settings_game.construction.build_on_slopes) {
 					return_cmd_error(STR_ERROR_LAND_SLOPED_IN_WRONG_DIRECTION);
 				}
-				if ((_invalid_tileh_slopes_road[0][tileh] & (pieces & ~entrance_piece)) != ROAD_NONE) {
+
+				/* Steep slopes behave the same as slopes with one corner raised. */
+				const Slope normalised_tileh = IsSteepSlope(tileh) ? SlopeWithOneCornerRaised(GetHighestSlopeCorner(tileh)) : tileh;
+
+				if ((_invalid_tileh_slopes_road[0][normalised_tileh & SLOPE_ELEVATED] & (pieces & ~entrance_piece)) != ROAD_NONE) {
 					return_cmd_error(STR_ERROR_LAND_SLOPED_IN_WRONG_DIRECTION);
 				}
 

src/tunnelbridge_cmd.cpp

@@ -1556,7 +1556,7 @@ static int GetSlopePixelZ_TunnelBridge(TileIndex tile, uint x, uint y)
 		if (5 <= pos && pos <= 10) return z;
 	} else { // IsBridge(tile)
 		if (IsRoadCustomBridgeHeadTile(tile)) {
-			return z + TILE_HEIGHT;
+			return z + TILE_HEIGHT + (IsSteepSlope(tileh) ? TILE_HEIGHT : 0);
 		}
 
 		DiagDirection dir = GetTunnelBridgeDirection(tile);
@@ -1959,7 +1959,11 @@ static CommandCost TerraformTile_TunnelBridge(TileIndex tile, DoCommandFlag flag
 		if (IsRoadCustomBridgeHeadTile(tile)) {
 			const RoadBits pieces = GetCustomBridgeHeadAllRoadBits(tile);
 			const RoadBits entrance_piece = DiagDirToRoadBits(direction);
-			if ((_invalid_tileh_slopes_road[0][tileh_new] & (pieces & ~entrance_piece)) != ROAD_NONE) {
+
+			/* Steep slopes behave the same as slopes with one corner raised. */
+			const Slope normalised_tileh_new = IsSteepSlope(tileh_new) ? SlopeWithOneCornerRaised(GetHighestSlopeCorner(tileh_new)) : tileh_new;
+
+			if ((_invalid_tileh_slopes_road[0][normalised_tileh_new & SLOPE_ELEVATED] & (pieces & ~entrance_piece)) != ROAD_NONE) {
 				return DoCommand(tile, 0, 0, flags, CMD_LANDSCAPE_CLEAR);
 			}
 		}