SparseArray code generated for simple array spreads

[DoctorGester]

[...[], ...[]]

https://typescripttolua.github.io/play#code/NoOjwXQGgAjFISA

This code uses lualib sparse arrays, even though there shouldn't be any need to, since arrays with nulls are not supported anyway. This might have a considerable impact on performance in hot loops.

[Perryvw]

This is intended behavior to deal with Lua behavior when using an unpack as an array literal member before the last element.

ie:

local a = { 1, 2, 3 }
local b = { 4, 5, 6 }
return { table.unpack(a), table.unpack(b) }

Will unexpectedly return {1, 4, 5, 6}.

While this will functionally work and result in the correct result, there are two issues with this:

• Like you said, the performance of this is questionable, there is not really a reason to use the expensive parse array push when a regular array push would do.
• We are making an extra copy of array in your example. It is already loading the result into a temporary array to do the array pushes on, but then once it's done we unpack and put it indo another array literal again, which is not required