LuaJIT asserts on negative non-existent indexes

[Arseny Vakhrushev <arseny.vakhrushev@...>]

Hello Mike and everyone!

I'm still playing with LuaJIT and have encountered a peculiar LuaJIT behavior.
If I call lua_is*() or lua_to*() function (lua_toboolean() or lua_touserdata() in my case) with a
negative non-existent index, assertion fails:

test2: lj_api.c:39: index2adr: Assertion `idx != 0 && -idx <= L->top - L->base' failed.

However, if I do the same with a positive but still non-existent index, assertion succeeds.
Like:

lua_State* L = luaL_newstate();
lua_toboolean(L, 10); // alright
lua_toboolean(L, -10); // <<-- assertion fails here

For lua_toboolean(), the manual states that

"It also returns 0 when called with a non-valid index."

So, I suppose I'm doing a legal thing trying to fetch a value at the end (-1) without checking a
stack size. Probably, it's not a big deal, and LuaJIT will still work fine with suppressed assertions,
but I really like this mode to make sure I do everything right.

Thanks,
Seny