RayCast2D and CollisionPolygon2D collisions are unreliable

[KoBeWi]

Godot version:
3.2.1

Issue description:
The Sprite becomes red when the raycast's is_colliding() returns true.

As you can see, raycast will detect collision only with the edge of polygon's quad and will fail inside.

Minimal reproduction project:
ReproductionProject.zip

EDIT:
Also tested with rectangle CollisionShape2D and it works correctly, so it's indeed a bug with polygons.

[bojidar-bg]

CollisionPolygon2D decomposes into multiple ConvexShape2D shapes when in "solids" mode:

godot/scene/2d/collision_polygon_2d.cpp

Lines 49 to 59 in 00949f0

	if (solids) {
	//here comes the sun, lalalala
	//decompose concave into multiple convex polygons and add them
	Vector<Vector<Vector2>> decomp = _decompose_in_convex();
	for (int i = 0; i < decomp.size(); i++) {
	Ref<ConvexPolygonShape2D> convex = memnew(ConvexPolygonShape2D);
	convex->set_points(decomp[i]);
	parent->shape_owner_add_shape(owner_id, convex);
	}
	} else {

To determine whether a shape intersects a raycast, CollisionSolver2DSW::solve_raycast uses Shape2DSW::intersects_segment:

godot/servers/physics_2d/collision_solver_2d_sw.cpp

Line 96 in 00949f0

if (!p_shape_B->intersect_segment(from, to, p, n)) {

Unfortunately, ConvexPolygonShape2DSW::intersect_segment does not check if both points are inside:

godot/servers/physics_2d/shape_2d_sw.cpp

Line 559 in 00949f0

if (!Geometry::segment_intersects_segment_2d(p_begin, p_end, points[i].pos, points[(i + 1) % point_count].pos, &res)) {

Probably such a check could be done after failing to intersect any of the lines using Geometry::is_point_in_polygon. Given that is_point_in_polygon does the check using any point definitely outside of the polygon, maybe some part of it can be merged with the loop in intersect_segment. Maybe even doing something like further_away = get_aabb().position - Vector2(1, 1), as any point outside the AABB of the polygon is definitely outside the polygon.

Edit: there might be faster ways to check for this, e.g. see https://stackoverflow.com/questions/1119627/how-to-test-if-a-point-is-inside-of-a-convex-polygon-in-2d-integer-coordinates

[Xrayez]

See also #38343, #35845.

[knightofiam]

Any workarounds?

[KoBeWi]

@knightofiam Area2D or KinematicBody2D and test_move()

[Houkime]

Still valid for v4.0.dev.custom_build [a0d800e]
That could be said to be a perfomance feature, but then the mode probably should not be called a solid.