vector2D:unpack() returning nil

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1

Why do I get different outputs is I use :unpack() directly, as opposed to putting it into variables?

It looks like 0 values are returned as nil.

local a = geo.vector2D.new(0, 1)
local b = geo.vector2D.new(1, 0)
local c = geo.vector2D.new(1, 1)

print('Packed', 'a', a, 'b', b, 'c', c)
print('Unpacked', 'a', a:unpack(), 'b', b:unpack(), 'c', c:unpack())
local ax, ay = a:unpack()
local bx, by = b:unpack()
local cx, cy = c:unpack()
print('Via variables a', ax, ay)
print('Via variables b', bx, by)
print('Via variables c', cx, cy)

Packed	a	(0.0, 1.0)	b	(1.0, 0.0)	c	(1.0, 1.0)
Unpacked	a	0.0	b	1.0	c	1.0	1.0
Via variables a	0.0	1.0
Via variables b	1.0	0.0
Via variables c	1.0	1.0
2

It's because of the way the Lua stack works -- vector2D:unpack() pushes two values on the stack, but a register can hold only one.
I think what's happening is that pdc is creating temporary registers to hold the result (i.e. the first return value) for vectors a and b, and optimizing away the one for c because it's the last argument to print. The bytecode's equivalent would look like:

local tempA = a:unpack()
local tempB = b:unpack()
print('Unpacked', 'a', tempA, 'b', tempB, 'c', c:unpack())
3

I wondered if it was some funny business like that. Thanks.