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The Lua 5.2 manual, Section 8, does not list the following 

    There is no local variable "arg" in a vararg function anymore.

One now has three options when writing code that must run correctly under 
both 5.1 and 5.2:

  1. Think in Lua 5.1 but adapt the code to be correct under 5.2 too.
  2. Think in Lua 5.2 but adapt the code to be correct under 5.1 too.
  3. Think in both and do explicit tests on _VERSION.  

Sample code to illustrate what I mean in the first two cases:

table.pack = table.pack  or  function(...) return arg end

arg = {'a','b','c','d'}

function test1(a,...) 
    local arg = table.pack(...)     -- not actually needed under 5.1

function test2(a,...)
    local arg = _G.arg              -- not actually needed under 5.2

test1(1,2,3,4)  --> 3 2 3 4
test2(1,2,3,4)  --> 4 a b c

Which is better?