Tail-call regression from Lua 5.3 to 5.4

[Jan Behrens <jbe-mlist@...>]

Hello,

I think there is a regression in regard to tail-calls from Lua 5.3 to
Lua 5.4. Consider the following program:

---
local tbl = { 1 }

local function func(n)
  if n >= 100 then
    print(debug.traceback())
    return
  end
  for k, v in pairs(tbl) do
    return func(n + 1)
  end
end

func(0)
---

On Lua 5.3.6, we get:

stack traceback:
        tailtest.lua:5: in function <tailtest.lua:3>
        (...tail calls...)
        tailtest.lua:13: in main chunk
        [C]: in ?

But on Lua 5.4.6, we get:

stack traceback:
        tailtest.lua:5: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        ...     (skipping 81 levels)
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in upvalue 'func'
        tailtest.lua:9: in local 'func'
        tailtest.lua:13: in main chunk
        [C]: in ?

I think this is because of the introduction of to-be-closed variables
in Lua 5.4 and the fourth argument to generic for-loops. Note, however,
that the example program does not use to-be-closed variables. Yet, the
generic for-loop prohibits the recursive call to be executed as a tail
call.

Is there any plan to fix this behavior?

Regards,
Jan Behrens

Attachment: tailtest.lua
Description: Binary data