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On 2018-12-20 10:19 p.m., Jonathan Goble wrote:
On Thu, Dec 20, 2018 at 6:56 PM Gabriel Bertilson <arboreous.philologist@gmail.com <mailto:arboreous.philologist@gmail.com>> wrote: 

    I went with

    if (0 < i && i <= 256 && !lua_isnone(L, lua_upvalueindex(i))) { ... }
But is this even necessary? i originates from the third return value from the generator function, which starts at 0 and only increments from there. Additionally, MAXUPVAL is 255, so itervarargs cannot be called with a second argument greater than 255, which means that i cannot be greater than 256 in the first place. In short, i is already guaranteed to be between 1 and 256 inclusive, and therefore guaranteed to be an acceptable argument to lua_upvalueindex, in the first place.
So unless I'm missing something here (I admit my Lua/C API skills are rusty), the if condition can be reduced back to just the original !lua_isnone call. 

local a,b,c = generator_fn()
a(b,-1)
a(b,1024)

?