When I read the Lua5.4(work2) source code, change of `isvalid`(https://github.com/lua/lua/blob/b95e46621873cfb460e1d11dcd153914d5d69f86/lapi.c#L42) confused me.
In my opinion, `ttisnil(&G(L)->nilvalue)` is always true, `(!ttisnil(o) || o != &G(L)->nilvalue)` can be simplied to `(o != &(G)->nilvalue)`.
Of course Roberto is always right. But I can't find my mistake.
It confused me for sereval days.