Point: Safer `operator-`

1. Require both operands to have the same signedness to avoid surprises.
2. Only allow unary negation on signed points.

Fixes #6409
Also fixes a potential bug in the sound position calculation.

Source/engine/point.hpp

@@ -90,6 +90,7 @@ struct PointOf {
 
 	constexpr PointOf<CoordT> operator-() const
 	{
+		static_assert(std::is_signed<CoordT>::value, "CoordT must be signed");
 		return { -x, -y };
 	}
 
@@ -193,6 +194,7 @@ constexpr PointOf<PointCoordT> operator+(PointOf<PointCoordT> a, Direction direc
 template <typename PointCoordT, typename OtherPointCoordT>
 constexpr DisplacementOf<PointCoordT> operator-(PointOf<PointCoordT> a, PointOf<OtherPointCoordT> b)
 {
+	static_assert(std::is_signed<PointCoordT>::value == std::is_signed<OtherPointCoordT>::value, "points must have the same signedness");
 	return { static_cast<PointCoordT>(a.x - b.x), static_cast<PointCoordT>(a.y - b.y) };
 }
 

Source/engine/sound_position.cpp

@@ -7,14 +7,13 @@ namespace devilution {
 
 bool CalculateSoundPosition(Point soundPosition, int *plVolume, int *plPan)
 {
-	const auto &playerPosition = MyPlayer->position.tile;
-	const auto delta = soundPosition - playerPosition;
+	const Point playerPosition { MyPlayer->position.tile };
+	const Displacement delta = soundPosition - playerPosition;
 
-	int pan = (delta.deltaX - delta.deltaY) * 256;
+	const int pan = (delta.deltaX - delta.deltaY) * 256;
 	*plPan = clamp(pan, PAN_MIN, PAN_MAX);
 
-	int volume = playerPosition.ApproxDistance(soundPosition);
-	volume *= -64;
+	const int volume = playerPosition.ApproxDistance(soundPosition) * -64;
 
 	if (volume <= ATTENUATION_MIN)
 		return false;

Source/missiles.cpp

@@ -1474,10 +1474,10 @@ void AddWarp(Missile &missile, AddMissileParameter &parameter)
 			}
 			app_fatal(StrCat("invalid leveltype", static_cast<int>(leveltype)));
 		};
-		Displacement triggerOffset = getTriggerOffset(trg);
+		const Displacement triggerOffset = getTriggerOffset(trg);
 		candidate += triggerOffset;
-		Displacement off = player.position.tile - candidate;
-		int distanceSq = off.deltaY * off.deltaY + off.deltaX * off.deltaX;
+		const Displacement off = Point { player.position.tile } - candidate;
+		const int distanceSq = off.deltaY * off.deltaY + off.deltaX * off.deltaX;
 		if (distanceSq < minDistanceSq) {
 			minDistanceSq = distanceSq;
 			tile = candidate;