Re: ${ac_fc_srcext-f} considered harmful

Could someone please apply this patch:

[PATCH] Ignore Set-But-Null `ac_fc_srcext` when setting `ac_ext`

This allows to reset `ac_fc_srcext` after `AC_FC_SRCEXT` was called:

```
save=$ac_fc_srcext
AC_FC_PP_SRCEXT([F])
ac_fc_srcext=$save
```

Which `AC_FC_PP_DEFINE()` also does and thus it breaks later
usage of `AC_LANG_PUSH(Fortran)`

* lib/autoconf/fortran.m4 (AC_LANG(Fortran)):
Do not consider a Set-But-Null `ac_fc_srcext` a valid value, and thus
use the default
`f` in this case too.
---
 lib/autoconf/fortran.m4 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/autoconf/fortran.m4 b/lib/autoconf/fortran.m4
index 17043c56..fbd64ea9 100644
--- a/lib/autoconf/fortran.m4
+++ b/lib/autoconf/fortran.m4
@@ -220,7 +220,7 @@ m4_define([AC_LANG_FUNC_LINK_TRY(Fortran 77)],
 # AC_LANG(Fortran)
 # ----------------
 AC_LANG_DEFINE([Fortran], [fc], [FC], [FC], [Fortran 77],
-[ac_ext=${ac_fc_srcext-f}
+[ac_ext=${ac_fc_srcext:-f}
 ac_compile='$FC -c $FCFLAGS $ac_fcflags_srcext conftest.$ac_ext
>&AS_MESSAGE_LOG_FD'
 ac_link='$FC -o conftest$ac_exeext $FCFLAGS $LDFLAGS
$ac_fcflags_srcext conftest.$ac_ext $LIBS >&AS_MESSAGE_LOG_FD'
 ac_compiler_gnu=$ac_cv_fc_compiler_gnu
-- 
2.17.0.591.ge996b92f55

On Sat, Dec 1, 2018 at 3:39 PM Bert Wesarg <[email protected]> wrote:
>
> ,Dear all,
>
> AC_LANG(Fortran) uses the above parameter expansion to set `ac_ext`, see here:
>
>     http://git.savannah.gnu.org/cgit/autoconf.git/tree/lib/autoconf/fortran.m4#n223
>
> Though I consider it harmful, because if $ac_fc_srcext is set but null
> the value of ac_ext is also null.
>
> That problem arises, if one wants to use the usual 'save=$var; : do
> something; var=$save' idiom. Which is done in fortran.m4 in
> AC_FC_PP_DEFINE here too:
>
>     http://git.savannah.gnu.org/cgit/autoconf.git/tree/lib/autoconf/fortran.m4#n1302
>
> It does:
>
> AC_LANG_PUSH([Fortran])dnl
> ac_fc_pp_define_srcext_save=$ac_fc_srcext
> AC_FC_PP_SRCEXT([F])
> AC_CACHE_CHECK(…)
> ac_fc_srcext=$ac_fc_pp_define_srcext_save
>
> If $ac_fc_srcext was unset before, its now set but null, thus the next
> ac_ext=${ac_fc_srcext-f} is null too.
>
> On a side note, AC_FC_PP_SRCEXT may also set $ac_fcflags_srcext, but
> that is not saved/restored by AC_FC_PP_DEFINE.
>
> Can someone please have a llok at my reasoning?
>
> Thanks.
>
> Best,
> Bert