[Mono] Plane intersect methods returning Vector3(0, 0, 0)

[wfowler1]

Godot version:
3.1.1
OS/device including version:
Windows 10
Issue description:
Per the documentation at https://docs.godotengine.org/en/3.1/classes/class_plane.html Plane.intersect_3, Plane.intersect_ray and Plane.intersect_segment will return null if no intersection is found. However, in C#/Mono, value types cannot be null, so instead these methods return Vector3(0, 0, 0). This isn't good, since (0, 0, 0) could be a valid intersection point but we have no way to know if it was the actual intersection or an invalid result. A better result might be to return Vector3(float.NaN, float.NaN, float.NaN).
Steps to reproduce:
Try to use any of these three methods from C# using data which won't actually intersect. You can use Intersect3 with planes:
(0, 0, 1), 0
(0, 0, 1), 1
[any other plane]

[Zylann]

Could we make it return Nullable<Vector3>?
Or return a bool with an out parameter, although that signature would need to be C# only.
Returning a class should be avoided because it would allocate on the heap and that's no good for realtime math.

[wfowler1]

Nullable<Vector3> would probably work as well.
As far as returning a bool with an out parameter, this is the approach Unity3D took for its raycasts and it works fairly well, though I'm not sure it's the cleanest implementation.
Really the main issue is knowing whether there actually was an intersection or not. However this is done, it shouldn't yield a result which could potentially be valid.

[aaronfranke]

Another option, if we want to use purely value types, is to return Vector3.Inf since infinity is easier to compare than NaN (NaN == NaN is always false).

I prefer nullable types to bool with out.

[NathanWarden]

I agree with Zylann about either for Nullable or returning bool with an out parameter since these seem to be basically self documenting. Nullable seems a little better since it's essentially a 1 to 1 equivalent with GDScript.

[neikeq]

I would prefer the bool and out way too.

[mwerezak]

Nullable or bool/out are the most idiosyncratic ways to do this in C# so it should probably be one of these two.

I can't think of any real pro/cons between the two so it seems like a purely stylistic difference. Returning a Vector3? (i.e. the nullable type) would more closely match the gdscript API.

The other ideas involving INF or NaN just seem awkward and weird as a longtime C# dev.

[aaronfranke]

@mwerezak Agreed, the PR I opened uses Vector3?, the suggestion of INF was just an idea. I prefer Vector3? over bool with out parameters personally.