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[url=https://github.com/coelckers/gzdoom/commit/4fa6678fc50f64a71daf7895d13bb3108977d421]Fixed.[/url] This was caused by [url=https://forum.zdoom.org/viewtopic.php?f=2&t=66180&p=1122315#p1122287]this bug[/url], but vectors also had some hardcoded register offset code which had to be modified for this to actually work.

[quote="Major Cooke"]The problem lies with structs, and vectors are such (for those who don't know).
[...]
If you pass in a null Line struct, it'll destroy the game with the null check. But it doesn't crash if it's not in/out.[/quote]
It's not really about being a struct, this is an issue that seems specific to vectors. The Line struct is also a native struct, which is always passed by pointer, not value, so it doesn't suffer from a lot of the problems "normal" structs do.
Also, please report that crash.

bool CheckLine(out Line ln)
{
	if (ln == null)
		return false;
	return true;
}

The problem lies with structs, and vectors are such (for those who don't know).

[code]bool CheckLine(out Line ln)
{
if (ln == null)
return false;
return true;
}
[/code]

If you pass in a null Line struct, it'll destroy the game with the null check. But it doesn't crash if it's not in/out.

[s][url=https://forum.zdoom.org/viewtopic.php?f=2&t=66180&p=1122287]This is caused by a bug in the ZScript compiler[/url][/s] Nevermind, I fucked up when debugging. Probably still a bug in the compiler, but not what I thought it was.

The check in the JIT is indeed one I added for sanity. It verifies that the input is what the code expects it to be. Either the input is invalid, or the code is based on assumptions that aren't always true. I assume its the former, but Graf should know for sure.

class BEPlayerBase : DoomPlayer
{
	private void CheckCursorMovement(out vector2 CursorPosition, int maxWidth, int maxHeight)
	{
		Console.Printf ("%f, %f", CursorPosition.X, CursorPosition.Y);
	}

	vector2 SomeGridCoords;

	override void Tick()
	{
		int buttons = player.original_cmd.buttons;
		int oldbuttons = player.original_oldbuttons;

		if (buttons & BT_ATTACK)
		{
			CheckCursorMovement((50, 25), 5, 5);
		}

		Super.Tick();
	}
}

Some testing with an alternate example indicates this likely isn't the JIT's fault at all, and is probably just a case of the JIT being less forgiving of broken bytecode.
[code]class BEPlayerBase : DoomPlayer
{
private void CheckCursorMovement(out vector2 CursorPosition, int maxWidth, int maxHeight)
{
Console.Printf ("%f, %f", CursorPosition.X, CursorPosition.Y);
}

vector2 SomeGridCoords;

override void Tick()
{
int buttons = player.original_cmd.buttons;
int oldbuttons = player.original_oldbuttons;

if (buttons & BT_ATTACK)
{
CheckCursorMovement((50, 25), 5, 5);
}

Super.Tick();
}
}[/code]
This alternate example causes the VM to crash with a Fatal Error window. Might include more details soon.

Edit: Further investigation shows that this is, indeed, (just as I thought) compiler fuckery:
[img]https://cdn.discordapp.com/attachments/630143990389538870/630710770199691264/unknown.png[/img]
More details soon maybe.

Crash message: "JIT: sfunc->NumArgs != argsPos || regd > sfunc->NumRegD || regf > sfunc->NumRegF || rega > sfunc->NumRegA"

The message above always shows up in the example below. Before isolating the faulty code, the game simply froze and crashed with no message, unless I initialized the variable inside the method with, say, (0, 0).

Example: https://drive.google.com/open?id=1MzRdl39JTUzN7piVpu2yUrXOWFuLTeZC