Re: AS_MESSAGE_FD

[Eric Blake]

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According to Ralf Wildenhues on 11/22/2008 10:43 AM:
>> -[m4_ifset([AS_MESSAGE_LOG_FD],
>> +[m4_ifval(AS_MESSAGE_LOG_FD,
> 
> If these kinds of changes are required in Autoconf code, it looks like
> user code might be affected, too.  Have you checked for third-party
> instances of this?  Also, is this an incompatibility to earlier
> semantics?

The semantics should be identical for third-party use; this is merely an
optimization change.

m4_ifset([AS_MESSAGE_LOG_FD], [a], [b]) expands to:

m4_ifdef([AS_MESSAGE_LOG_FD],
  [m4_if(m4_defn([AS_MESSAGE_LOG_FD]), [], [b], [a])], [b])

But since AS_MESSAGE_LOG_FD (if defined) must expand to either a number or
the empty string, then it should generally be safe to expand
AS_MESSAGE_LOG_FD rather than going through m4_defn (well, it is possible
to have AS_MESSAGE_LOG_FD defined as [$[3]], as in my recent as_fn_error,
but even that string is still safe for immediate expansion; and there were
other places in m4sh that perform AS_MESSAGE_LOG_FD expansion).  Also, by
guaranteeing a definition from m4sh (rather than the previous semantics
where it was undefined if the user didn't supply it), the m4_ifdef portion
of m4_ifset is wasted effort.  Which leaves us with the faster:

m4_ifval(AS_MESSAGE_LOG_FD, [a], [b])

expanding to

m4_if(AS_MESSAGE_LOG_FD, [], [b], [a])

- --
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden
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