Re: AC_USE_SYSTEM_EXTENSIONS should be AC_DEFUN_ONCEd

[Ralf Wildenhues]

* Eric Blake wrote on Mon, Jan 26, 2009 at 10:16:30PM CET:
> Subject: [PATCH] Use AC_DEFUN_ONCE more liberally.
> 
> * lib/autoconf/programs.m4 (AC_PROG_MAKE_SET): Switch to
> AC_DEFUN_ONCE, since this is a one-shot macro.
> (AC_PROG_AWK, AC_PROG_GREP, AC_PROG_EGREP, AC_PROG_FGREP)
> (AC_PROG_INSTALL, AC_PROG_MKDIR_P, AC_PROG_LN_S, AC_PROG_RANLIB)
> (AC_PROG_SED, AC_PROG_YACC): Likewise.
> * lib/autoconf/headers.m4 (AC_HEADER_ASSERT): Likewise.
> * lib/autoconf/lang.m4 (AC_LANG_WERROR): Likewise.
> * lib/autoconf/c.m4 (AC_PROG_CC_C_O, AC_PROG_CPP)
> (AC_PROG_CPP_WERROR, AC_PROG_CC_STDC, AC_PROG_CC_C89)
> (AC_PROG_CC_C99, AC_PROG_GCC_TRADITIONAL, AC_PROG_CXXCPP)
> (AC_PROG_CXX_C_O, AC_PROG_OBJCPP): Likewise.
> * lib/autoconf/fortran.m4 (AC_PROG_F77_C_O, AC_PROG_FC_C_O)
> (AC_F77_LIBRARY_LDFLAGS, AC_FC_LIBRARY_LDFLAGS, AC_F77_MAIN)
> (AC_FC_MAIN, AC_F77_WRAPPERS, AC_FC_WRAPPERS): Likewise.
> * lib/autoconf/libs.m4 (AC_PATH_X, AC_PATH_XTRA): Likewise.
> * lib/autoconf/specific.m4 (AC_SYS_INTERPRETER)
> (AC_SYS_LARGEFILE, AC_SYS_LONG_FILE_NAMES)
> (AC_USE_SYSTEM_EXTENSIONS): Likewise.
> * lib/autoconf/erlang.m4 (AC_ERLANG_SUBST_ROOT_DIR)
> (AC_ERLANG_SUBST_LIB_DIR, AC_ERLANG_SUBST_INSTALL_LIB_DIR):
> Likewise.
> * lib/autoconf/general.m4 (AC_CANONICAL_BUILD)
> (AC_CANONICAL_HOST, AC_CANONICAL_TARGET, AC_PRESERVE_HELP_ORDER)
> (AC_DISABLE_OPTION_CHECKING):

So all of these macros are now incompatibly changed, silently.
This looks like you're inventing a whole new language, where
plain shell-conditional invocation of macros is completely forbidden,
and many tests cannot be conditionalized at all any more.  I wouldn't
even know how to systematically check for all the little
incompatibilites this can cause.

Is there no way this can be fixed with less churn?  If not, then I think
all these macros (and what's more, any macros that require any of the
above) should be marked as nonobvious-to-use in the manual, e.g. a set
of macros in Automake and Libtool, too.

Sorry to sound so negative, but this seems quite dangerous to me.

Thanks,
Ralf