CC @mihe

When using Jolt Physics you won't be able to use ConcavePolygonShape3D with Area3D at the moment.

I honestly can't remember if this was a deliberate deviation on my part (and I just forgot about it) or if I just assumed that this didn't work in Godot Physics either, given the warning in the documentation of Area3D about using concave shapes with it:

Warning: Using a ConcavePolygonShape3D inside a CollisionShape3D child of this node (created e.g. by using the Create Trimesh Collision Sibling option in the Mesh menu that appears when selecting a MeshInstance3D node) may give unexpected results, since this collision shape is hollow. If this is not desired, it has to be split into multiple ConvexPolygonShape3Ds or primitive shapes like BoxShape3D, or in some cases it may be replaceable by a CollisionPolygon3D.

As hinted at there you can end up with some strange results with this setup, as you'll only ever register overlaps if the other shape happens to intersect with one of the triangles, and only if it's a "shallow" enough intersection to not be a backface hit. I'm sure there are probably other odd artefacts with this setup as well.

What you might want instead is (as mentioned above) to use either a single or multiple ConvexPolygonShape3D, both of which you can generate from a MeshInstance3D from the viewport toolbar.

Either way, using any concave shape won't work with how I've implemented Area3D in the Jolt integration, as it's done using a kinematic body under the hood, which only supports convex shapes (i.e. solid shapes).

Oh right, concave shapes are a common limitation in physics engines but the error message got me confused. It could be changed to clarify that the shape type is not supported.

the error message got me confused. It could be changed to clarify that the shape type is not supported.

Yes, definitely. The current error was not intentional. I'll see about adding something more useful instead, if only just as a hold-over.

It seems I need to correct myself here.

When taking a closer look at using ConcavePolygonShape3D for Area3D, with the way things are now, it seems it already "works", much to my surprise, in terms of it being able to detect things like overlaps with other (convex) bodies. So all that needed to be done here was provide Area3D with some default mass properties, just to prevent the assertion mentioned in the issue description here, and that was that.

With that said, the thing about overlaps being janky still applies. You still ideally shouldn't use a ConcavePolygonShape3D for an Area3D, and the warning in the documentation still applies, but at least we're at some kind of parity with Godot Physics already.

You'll also be met with a lot of errors (in dev_build=yes) saying "Unsupported shape pair" if you end up having your ConcavePolygonShape3D Area3D overlap with another ConcavePolygonShape3D of any kind, whether it be another Area3D or a StaticBody3D. This is something you'll see with Godot Physics as well though, but there it's a warning instead, and only emitted once during the lifetime of the application.