Fix assertion failure when upgrading dual road/tram bridge

In case where the other road/tram type does not extend across
the bridge, but is present on the upgrade tile.

src/bridge_map.h

@@ -148,20 +148,13 @@ static inline void MakeBridgeRamp(TileIndex t, Owner o, BridgeType bridgetype, D
  * @param d          the direction this ramp must be facing
  * @param road_rt    the road type of the bridge
  * @param tram_rt    the tram type of the bridge
- * @param upgrade    whether the bridge is an upgrade instead of a totally new bridge
  */
-static inline void MakeRoadBridgeRamp(TileIndex t, Owner o, Owner owner_road, Owner owner_tram, BridgeType bridgetype, DiagDirection d, RoadType road_rt, RoadType tram_rt, bool upgrade)
+static inline void MakeRoadBridgeRamp(TileIndex t, Owner o, Owner owner_road, Owner owner_tram, BridgeType bridgetype, DiagDirection d, RoadType road_rt, RoadType tram_rt)
 {
-	// Backup custom bridgehead data.
-	uint custom_bridge_head_backup = GB(_m[t].m2, 0, 8);
-
 	MakeBridgeRamp(t, o, bridgetype, d, TRANSPORT_ROAD);
 	SetRoadOwner(t, RTT_ROAD, owner_road);
 	if (owner_tram != OWNER_TOWN) SetRoadOwner(t, RTT_TRAM, owner_tram);
 	SetRoadTypes(t, road_rt, tram_rt);
-
-	// Restore custom bridgehead data if we're upgrading an existing bridge.
-	if (upgrade) SB(_m[t].m2, 0, 8, custom_bridge_head_backup);
 }
 
 /**

src/tunnelbridge_cmd.cpp

@@ -639,10 +639,17 @@ CommandCost CmdBuildBridge(TileIndex end_tile, DoCommandFlag flags, uint32 p1, u
 
 					Owner owner_road = hasroad ? GetRoadOwner(t, RTT_ROAD) : company;
 					Owner owner_tram = hastram ? GetRoadOwner(t, RTT_TRAM) : company;
-					MakeRoadBridgeRamp(t, owner, owner_road, owner_tram, bridge_type, d, road_rt, tram_rt, is_upgrade);
+
 					if (is_upgrade) {
-						if (road_rt != INVALID_ROADTYPE) SetCustomBridgeHeadRoadBits(t, RTT_ROAD, GetCustomBridgeHeadRoadBits(t, RTT_ROAD) | DiagDirToRoadBits(d));
-						if (tram_rt != INVALID_ROADTYPE) SetCustomBridgeHeadRoadBits(t, RTT_TRAM, GetCustomBridgeHeadRoadBits(t, RTT_TRAM) | DiagDirToRoadBits(d));
+						RoadBits road_bits = GetCustomBridgeHeadRoadBits(t, RTT_ROAD);
+						RoadBits tram_bits = GetCustomBridgeHeadRoadBits(t, RTT_TRAM);
+						if (RoadTypeIsRoad(roadtype)) road_bits |= DiagDirToRoadBits(d);
+						if (RoadTypeIsTram(roadtype)) tram_bits |= DiagDirToRoadBits(d);
+						MakeRoadBridgeRamp(t, owner, owner_road, owner_tram, bridge_type, d, road_rt, tram_rt);
+						if (road_rt != INVALID_ROADTYPE) SetCustomBridgeHeadRoadBits(t, RTT_ROAD, road_bits);
+						if (tram_rt != INVALID_ROADTYPE) SetCustomBridgeHeadRoadBits(t, RTT_TRAM, tram_bits);
+					} else {
+						MakeRoadBridgeRamp(t, owner, owner_road, owner_tram, bridge_type, d, road_rt, tram_rt);
 					}
 				};
 				make_bridge_ramp(tile_start, dir);