Fix: Resolve cargo-types of default vehicles via their cargo label.

Default vehicles now behave as if they had a cargo translation table. This fixes default vehicles carrying seemingly random cargos, if NewGRF industry sets are present.
This behavior is disabled, when a NewGRF touches any of the cargo-type or refitting properties. In that case it's up to the NewGRF to define its own cargo translation table.

src/cargotype.cpp

@@ -77,6 +77,28 @@ void SetupCargoForClimate(LandscapeID l)
 	}
 }
 
+/**
+ * Get the cargo ID of a default cargo, if present.
+ * @param l Landscape
+ * @param ct Default cargo type.
+ * @return ID number if the cargo exists, else #CT_INVALID
+ */
+CargoID GetDefaultCargoID(LandscapeID l, CargoType ct)
+{
+	assert(l < lengthof(_default_climate_cargo));
+
+	if (ct == CT_INVALID) return CT_INVALID;
+
+	assert(ct < lengthof(_default_climate_cargo[0]));
+	CargoLabel cl = _default_climate_cargo[l][ct];
+	/* Bzzt: check if cl is just an index into the cargo table */
+	if (cl < lengthof(_default_cargo)) {
+		cl = _default_cargo[cl].label;
+	}
+
+	return GetCargoIDByLabel(cl);
+}
+
 /**
  * Get the cargo ID by cargo label.
  * @param cl Cargo type to get.

src/cargotype.h

@@ -177,6 +177,7 @@ extern CargoTypes _standard_cargo_mask;
 void SetupCargoForClimate(LandscapeID l);
 CargoID GetCargoIDByLabel(CargoLabel cl);
 CargoID GetCargoIDByBitnum(uint8 bitnum);
+CargoID GetDefaultCargoID(LandscapeID l, CargoType ct);
 
 void InitializeSortedCargoSpecs();
 extern std::vector<const CargoSpec *> _sorted_cargo_specs;

src/newgrf.cpp

@@ -8768,6 +8768,12 @@ static void CalculateRefitMasks()
 		EngineInfo *ei = &e->info;
 		bool only_defaultcargo; ///< Set if the vehicle shall carry only the default cargo
 
+		/* If the NewGRF did not set any cargo properties, we apply default cargo translation. */
+		if (_gted[engine].defaultcargo_grf == nullptr) {
+			/* Translate cargo_type using the original climate-specific cargo table. */
+			ei->cargo_type = GetDefaultCargoID(_settings_game.game_creation.landscape, static_cast<CargoType>(ei->cargo_type));
+		}
+
 		/* Did the newgrf specify any refitting? If not, use defaults. */
 		if (_gted[engine].refittability != GRFTempEngineData::UNSET) {
 			CargoTypes mask = 0;
@@ -8814,7 +8820,7 @@ static void CalculateRefitMasks()
 		}
 
 		/* Clear invalid cargoslots (from default vehicles or pre-NewCargo GRFs) */
-		if (!HasBit(_cargo_mask, ei->cargo_type)) ei->cargo_type = CT_INVALID;
+		if (ei->cargo_type != CT_INVALID && !HasBit(_cargo_mask, ei->cargo_type)) ei->cargo_type = CT_INVALID;
 
 		/* Ensure that the vehicle is either not refittable, or that the default cargo is one of the refittable cargoes.
 		 * Note: Vehicles refittable to no cargo are handle differently to vehicle refittable to a single cargo. The latter might have subtypes. */