Dangerous tiles hurt Tux even when they are blocked off

[HybridDog]

SuperTux version: v0.5.1-1937-g4d1c419ea
System information: lubuntu

Expected behavior

Tux does not get hurt when he jumps onto the rock

Actual behavior

The spikes kill Tux although they are blocked by a rock object, which is as high as one tile

Steps to reproduce actual behavior

See the screenshot, jump onto the rock

[tobbi]

I feel like this is a collision rect problem where the collisiion rect of the spikes is maybe 1 or 2px larger than that of the block.

[HybridDog]

If I understand the collision system correctly, it firstly moves objects to target positions, which depend on the objects' velocities. Then it tests if the object at the target position intersects with a tile, and after that it tests collision with other objects.
http://github.com/SuperTux/supertux/blob/3827b0555c294e69f100525f7360b2b652362f4d/src/collision/collision_system.cpp#L494
If it works this way, Tux would also get hurt when his velocity is high enough and the rock is bigger than the spikes.

[tobbi]

That doesn't seem to be a good idea. If that's indeed the case, we'd have to change it.

[HybridDog]

Here's a similar problem:
When I press left while falling right of a wall and then release left before the end of the wall, Tux nonetheless gets hurt by the spike:
https://user-images.githubusercontent.com/3192173/105752916-2f40ec00-5f48-11eb-99d6-3bc4f7ea88b0.mp4