Re: Lua 5.5: LUA_COMPAT_LOOPVAR does not fully restore the previous behavior

> I would like to report what I believe is an oversight (or at least an
> under-documented limitation) in the new LUA_COMPAT_LOOPVAR option in
> Lua 5.5.
> 
> The comment in luaconf.h states:
> 
>     @@ LUA_COMPAT_LOOPVAR makes for-loop control variables not read-only,
>     ** as they were in previous versions.
> 
> The phrase "as they were in previous versions" suggests that enabling
> this option is supposed to restore the Lua 5.4 behavior. In practice,
> the option only flips the variable kind from RDKCONST to VDKREG (see
> lparser.c, around the LOOPVARKIND macro). The compiler stops rejecting
> the assignment, but the underlying single-slot design of 5.5 is
> unchanged: the user-visible loop variable and the internal control
> variable share the same stack register (e.g. ra+3 for a generic for).
> As a consequence, any assignment to the loop variable inside the body
> corrupts the value that OP_TFORCALL passes back to the iterator on the
> next iteration.

You are right. Thanks for the feedback.

-- Roberto

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