Re: Possible bug: AC_USE_SYSTEM_EXTENSIONS ignores "AS_IF" conditionals

[Gavin Smith]

I got it down to the use of AC_DEFUN_ONCE in the definition of
AC_USE_SYSTEM_EXTENSIONS. If configure.ac is

AC_DEFUN_ONCE([AC_USE_SYSTEM_EXTENSIONS_TEST],
[ echo here
])# AC_USE_SYSTEM_EXTENSIONS

AC_PREREQ([2.63])
AC_INIT([bug], [0.01])
# XXXXX 111111111111111111111111111111111111111111111111111111111111111111111
AS_IF([test 0 = 1], [AC_USE_SYSTEM_EXTENSIONS_TEST])
# XXXXXX 22222222222222222222222222222222222222222222222222222222222222222222
AC_OUTPUT

the following appears in configure:

# XXXXX 111111111111111111111111111111111111111111111111111111111111111111111
 echo here

if test 0 = 1; then :

fi
# XXXXXX 22222222222222222222222222222222222222222222222222222222222222222222

If AC_DEFUN is used instead, this becomes

# XXXXX 111111111111111111111111111111111111111111111111111111111111111111111
if test 0 = 1; then :
   echo here

fi
# XXXXXX 22222222222222222222222222222222222222222222222222222222222222222222

I found there was a mention of this "hoisting" in a discussion:
http://lists.gnu.org/archive/html/autoconf-patches/2009-01/msg00044.html.

> Am I going about this the wrong way?
>
> Suggestions welcome.

I haven't investigated or thought about whether this is the right behaviour.