Fix artic engines with no capacity on leading part in build window

Engines were being shown as not having any capacity overall

src/build_vehicle_gui.cpp

@@ -1085,6 +1085,7 @@ void TestedEngineDetails::FillDefaultCapacities(const Engine *e)
 		this->all_capacities[this->cargo] = this->capacity;
 		this->all_capacities[CT_MAIL] = this->mail_capacity;
 	}
+	if (this->all_capacities.GetCount() == 0) this->cargo = CT_INVALID;
 }
 
 /**
@@ -1521,7 +1522,7 @@ struct BuildVehicleWindow : BuildVehicleWindowBase {
 		if (this->sel_engine == INVALID_ENGINE) return;
 
 		const Engine *e = Engine::Get(this->sel_engine);
-		if (!e->CanCarryCargo()) {
+		if (!e->CanPossiblyCarryCargo()) {
 			this->te.cost = 0;
 			this->te.cargo = CT_INVALID;
 			this->te.all_capacities.Clear();
@@ -2377,7 +2378,7 @@ struct BuildVehicleWindowTrainAdvanced final : BuildVehicleWindowBase {
 		if (state.sel_engine == INVALID_ENGINE) return;
 
 		const Engine *e = Engine::Get(state.sel_engine);
-		if (!e->CanCarryCargo()) {
+		if (!e->CanPossiblyCarryCargo()) {
 			state.te.cost = 0;
 			state.te.cargo = CT_INVALID;
 			state.te.all_capacities.Clear();
@@ -2418,7 +2419,7 @@ struct BuildVehicleWindowTrainAdvanced final : BuildVehicleWindowBase {
 		}
 
 		/* Purchase test was not possible or failed, fill in the defaults instead. */
-		state.te.cost     = 0;
+		state.te.cost = 0;
 		state.te.FillDefaultCapacities(e);
 	}
 

src/engine.cpp

@@ -186,6 +186,35 @@ bool Engine::CanCarryCargo() const
 	return this->GetDefaultCargoType() != CT_INVALID;
 }
 
+bool Engine::CanPossiblyCarryCargo() const
+{
+	if (this->IsGroundVehicle() && HasBit(this->info.callback_mask, CBM_VEHICLE_ARTIC_ENGINE)) return true;
+
+	switch (this->type) {
+		case VEH_TRAIN:
+			if (HasBit(this->cb36_properties_used, PROP_TRAIN_CARGO_CAPACITY)) return true;
+			break;
+
+		case VEH_ROAD:
+			if (HasBit(this->cb36_properties_used, PROP_ROADVEH_CARGO_CAPACITY)) return true;
+			break;
+
+		case VEH_SHIP:
+			if (HasBit(this->cb36_properties_used, PROP_SHIP_CARGO_CAPACITY)) return true;
+			break;
+
+		case VEH_AIRCRAFT:
+			if (HasBit(this->cb36_properties_used, PROP_AIRCRAFT_PASSENGER_CAPACITY)) return true;
+			if (HasBit(this->cb36_properties_used, PROP_AIRCRAFT_MAIL_CAPACITY)) return true;
+			break;
+
+		default:
+			NOT_REACHED();
+	}
+
+	return this->CanCarryCargo();
+}
+
 
 /**
  * Determines capacity of a given vehicle from scratch.

src/engine_base.h

@@ -114,6 +114,7 @@ struct Engine : EnginePool::PoolItem<&_engine_pool> {
 	uint DetermineCapacity(const Vehicle *v, uint16 *mail_capacity = nullptr) const;
 
 	bool CanCarryCargo() const;
+	bool CanPossiblyCarryCargo() const;
 
 	/**
 	 * Determines the default cargo capacity of an engine for display purposes.