[sr #110987] AC_PROG_CC_C_O + AC_LANG([C++]) combines C-specific compiler options with C++ source

[Zack Weinberg]

URL:
  <https://savannah.gnu.org/support/?110987>

                 Summary: AC_PROG_CC_C_O + AC_LANG([C++]) combines C-specific
compiler options with C++ source
                   Group: Autoconf
               Submitter: zackw
               Submitted: Thu 21 Dec 2023 07:14:49 PM UTC
                Priority: 5 - Unprioritized
                Severity: 2 - Minor
                  Status: Confirmed
                 Privacy: Public
             Assigned to: None
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
        Operating System: None


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Follow-up Comments:


-------------------------------------------------------
Date: Thu 21 Dec 2023 07:14:49 PM UTC By: Zack Weinberg <zackw>
A configure.ac like this


AC_INIT
AC_LANG([C++])
AC_PROG_CC_C_O
...


may (depending on exactly which compiler you have) run a compilation command
that combines C-specific options with a C++ source file.  Some compilers
object to this:


configure:2575: checking for C compiler version
configure:2584: gcc --version >&5
Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin13.4.0
...
configure:3008: gcc -std=gnu11 -c -g -O2  conftest.c >&5
configure:3008: $? = 0
configure:3030: result: -std=gnu11
configure:3146: checking whether gcc -std=gnu11 and cc understand -c and -o
together
configure:3178: gcc -std=gnu11 -c conftest.cpp -o conftest2.o >&5
error: invalid argument '-std=gnu11' not allowed with 'C++/ObjC++'


and that in turn causes the test to get the wrong answer, thinking the C++
compiler does *not* support -c and -o together, when it does.

AC_PROG_CC_C_O is specifically a test of the C compiler. There's another macro
AC_PROG_CXX_C_O that does the same test on the C++ compiler (and two more for
Fortran compilers).  They should all be
either refusing to execute under the wrong AC_LANG setting, or ignoring the
AC_LANG setting; not invoking the C compiler, using C-specific options, on a
C++ source file.

Originally reported by Sevan Janiyan.







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