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Peter Cawley wrote:

_ENV is always the first upvalue of a chunk (i.e. a function created
by passing a piece of non-bytecode source code to lua_load), but this
is not true for functions in general. Functions in general may or may
not have an upvalue called _ENV, which may or may not be a shared
upvalue, and if they do have an _ENV upvalue, it doesn't have to be
the 1st upvalue.

Thanks! This clarifies why my attempt didn't work!

The specific use case I'm trying to tackle is the following (simplified):

local function Runner(func)

local env = {}
-- env is populated here with entries, e.g.:
env.Log = function(msg)
  -- store away the message
-- ...
local oldenv = getfenv(func)
-- set a metatable so that oldenv is still
-- visible, but entries in env "override" those
-- in oldenv
setmetatable( env, { __index = oldenv } )
setfenv(func, env)


-- here env is used with
-- any possible modification done
-- by running func
-- ...

Then Runner is called like this:

Runner( function()
  -- anonymous function which performs some task
  -- using the facilities provided by "Runner", e.g.:
  Log 'initializing the transmogryfier...'
 -- ...


So I hope there is a way to replace setfenv/getfenv easily in 5.2 (at least for this use case), otherwise I have to rewrite those scripts in a different way.