[sr #109676] only one "checking whether the AC_LANG compiler works"

URL:
  <https://savannah.gnu.org/support/?109676>

                 Summary: only one "checking whether the AC_LANG compiler
works"
                 Project: Autoconf
            Submitted by: None
            Submitted on: Sat 04 May 2019 07:44:13 PM UTC
                Category: None
                Priority: 5 - Normal
                Severity: 3 - Normal
                  Status: None
                 Privacy: Public
             Assigned to: None
        Originator Email: [email protected]
             Open/Closed: Open
         Discussion Lock: Any
        Operating System: GNU/Linux

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Details:

Consider this simple configure.ac file.
AC_INIT
AC_PROG_CC
AC_PROG_CXX

AC_LANG(C)
AC_COMPILE_IFELSE([AC_LANG_SOURCE([
int i = 0;
])],
    [AC_MSG_RESULT([yes])],
    [AC_MSG_ERROR([int not supported])])
AC_LANG(C++)
AC_COMPILE_IFELSE([AC_LANG_SOURCE([
int i = 0;
])],
    [AC_MSG_RESULT([yes])],
    [AC_MSG_ERROR([int not supported])])

If I run autoconf 2.69 to create a configure script, and then run it with a
broken C compiler, I get a useful error.
rohan:2097$ CC=/bin/broken ./configure
checking for gcc... /bin/broken
checking whether the C compiler works... no
configure: error: in `/home/jimw/tmp':
configure: error: C compiler cannot create executables
See `config.log' for more details
rohan:2098$ 

If I run it with a broken C++ compiler, I get a confusing error.
rohan:2098$ CXX=/bin/broken ./configure
checking for gcc... gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables... 
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
checking for gcc option to accept ISO C89... none needed
checking whether we are using the GNU C++ compiler... no
checking whether /bin/broken accepts -g... no
yes
configure: error: int not supported
rohan:2099$ 

The problem here is that the check to verity whether a compiler works is
buried in _AC_COMPILER_EXEEXT_DEFAULT in lang.m4 which is used in c.m4 via
m4_expand_once([_AC_COMPILER_EXEEXT])[]dnl
so only the very first AC_PROG_C* call gets checked.  Most linux distros will
let you install a C compiler without installing a C++ compiler.  This has
resulted in some user confusion with the GCC configure script.  See for
instance
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63509
and
https://github.com/riscv/riscv-gnu-toolchain/issues/431

We could work around the problem in GCC by calling AC_PROG_CXX before
AC_PROG_CC, but it would be nice if autoconf created a configure script that
verified that both compilers work instead of just the first one.





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