For the 5.0 target, we are currently transpiling the SparseArraySpread lualib feature as:

local function __TS__SparseArraySpread(sparseArray)
    return unpack(nil, sparseArray, 1, sparseArray.sparseLength)
end

The nil argument causes runtime errors and should not be there; unpack() takes 3 arguments. Or, rather, it does in all versions of Lua except 5.0, which does not support the optional i and j arguments that specify the start and end of the unpacked slice.

So we need to add a polyfill that supports i and j, and also convinces the type checker that it's okay to call unpack() without a leading nil.