AC_USE_SYSTEM_EXTENSIONS should be AC_DEFUN_ONCEd

[Bruno Haible]

Hi,

Assuming the new semantics of AC_DEFUN_ONCE proposed by Paolo Bonzini and
Eric Blake, I think the macro AC_USE_SYSTEM_EXTENSIONS should be defined
through AC_DEFUN_ONCE.

Here are four scenarios I observe with autoconf 2.63:

------------------------------------- 1 -------------------------------------

$ cat > configure.ac
AC_INIT
AC_CONFIG_SRCDIR([configure.ac])

AC_PROG_CC

AC_USE_SYSTEM_EXTENSIONS
AC_USE_SYSTEM_EXTENSIONS

AC_OUTPUT
$ autoconf
configure.ac:7: warning: AC_COMPILE_IFELSE was called before 
AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:386: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:7: the top level
configure.ac:7: warning: AC_RUN_IFELSE was called before 
AC_USE_SYSTEM_EXTENSIONS

-----------------------------------------------------------------------------

This shows that AC_USE_SYSTEM_EXTENSIONS may not be invoked twice. Therefore
the 2.63 doc should recommend to AC_REQUIRE it.

------------------------------------- 2 -------------------------------------

$ cat > configure.ac
AC_INIT
AC_CONFIG_SRCDIR([configure.ac])

AC_PROG_CC

AC_USE_SYSTEM_EXTENSIONS
AC_GNU_SOURCE

AC_OUTPUT
$ autoconf
configure.ac:7: warning: AC_COMPILE_IFELSE was called before 
AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:386: AC_USE_SYSTEM_EXTENSIONS is expanded from...
../../lib/autoconf/specific.m4:332: AC_GNU_SOURCE is expanded from...
configure.ac:7: the top level
configure.ac:7: warning: AC_RUN_IFELSE was called before 
AC_USE_SYSTEM_EXTENSIONS

------------------------------------- 3 -------------------------------------

$ cat > configure.ac
AC_INIT
AC_CONFIG_SRCDIR([configure.ac])

AC_PROG_CC

AC_GNU_SOURCE
AC_USE_SYSTEM_EXTENSIONS

AC_OUTPUT
$ autoconf
configure.ac:7: warning: AC_COMPILE_IFELSE was called before 
AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:386: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:7: the top level
configure.ac:7: warning: AC_RUN_IFELSE was called before 
AC_USE_SYSTEM_EXTENSIONS

-----------------------------------------------------------------------------

This shows that AC_USE_SYSTEM_EXTENSIONS and AC_GNU_SOURCE cannot be both
invoked, in either order. The bug in 2.63 is IMO that AC_GNU_SOURCE is
defined to *invoke* AC_USE_SYSTEM_EXTENSIONS, rather than to *require* it.

------------------------------------- 4 -------------------------------------

$ cat > configure.ac
AC_INIT
AC_CONFIG_SRCDIR([configure.ac])

AC_PROG_CC

dnl Make sure we see all GNU and Solaris extensions.
AC_DEFUN([xy_EARLY],
[
  AC_REQUIRE([AC_USE_SYSTEM_EXTENSIONS])
  AC_REQUIRE([AC_GNU_SOURCE])
])
xy_EARLY

AC_OUTPUT
$ autoconf
configure.ac:12: warning: AC_COMPILE_IFELSE was called before 
AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:386: AC_USE_SYSTEM_EXTENSIONS is expanded from...
../../lib/autoconf/specific.m4:332: AC_GNU_SOURCE is expanded from...
configure.ac:7: xy_EARLY is expanded from...
configure.ac:12: the top level
configure.ac:12: warning: AC_RUN_IFELSE was called before 
AC_USE_SYSTEM_EXTENSIONS

-----------------------------------------------------------------------------

This shows that even *requiring* both AC_USE_SYSTEM_EXTENSIONS and
AC_GNU_SOURCE does not work if the AC_REQUIREs are in the wrong order.

For 2.64, defining AC_USE_SYSTEM_EXTENSIONS through AC_DEFUN_ONCE should fix
all these problems, IMO. But feel free to add these 4 test cases to the test
suite nevertheless :-)

Bruno