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From: "Tim Van Holder" <tim DOT van DOT holder AT pandora DOT be>
To: "'Laurynas Biveinis'" <lauras AT softhome DOT net>, <bug-autoconf AT gnu DOT org>
Cc: <djgpp AT delorie DOT com>
Subject: Re: AC_PROG_CXX does not work correctly after AC_PROG_CC
Date: Sat, 20 Apr 2002 15:39:05 +0200
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> and, among others, will substitute CXX and CXXFLAGS variables in
> output files.  So it does not detect that in fact there is no working
> C++ compiler.  If I remove AC_PROG_CC in this configure script, then
> I get correct error message:
> checking for C++ compiler default output... configure: error: C++
> compiler cannot create executables
> But such a check is missing if there is AC_PROG_CC before AC_PROG_CXX.
> 
> I've verified it with DJGPP port of Autoconf 2.50 and Cygwin port of
> Autoconf 2.52.
> 
> Is there any fix or workaround? Please CC me on replies and TIA ;)

Actually, Laurynas, it seems that this is perfectly correct behaviour.

From the autoconf docs for AC_PROG_CXX:

    Determine a C++ compiler to use.  Check if the environment variable
    `CXX' or `CCC' (in that order) is set; if so, then set output
    variable `CXX' to its value.

    Otherwise, if the macro is invoked without an argument, then
    search for a C++ compiler under the likely names (first `g++' and
    `c++' then other names).  If none of those checks succeed, then as
    a last resort set `CXX' to `g++'.

    This macro may, however, be invoked with an optional first argument
    which, if specified, must be a space separated list of C++
    compilers to search for.  This just gives the user an opportunity
    to specify an alternative search list for the C++ compiler.  For
    example, if you didn't like the default order, then you could
    invoke `AC_PROG_CXX' like this:

         AC_PROG_CXX(cl KCC CC cxx cc++ xlC aCC c++ g++ egcs gcc)

    If using the GNU C++ compiler, set shell variable `GXX' to `yes'.
    If output variable `CXXFLAGS' was not already set, set it to `-g
    -O2' for the GNU C++ compiler (`-O2' on systems where G++ does not
    accept `-g'), or `-g' for other compilers.

Now as a side-effect, the first compiler check (PROG_CC or PROG_CXX)
will also try to determine the executable extension using that compiler.
It is that test that bombs out if you only use PROG_CXX (or use it
first)
on a system without a C++ compiler.
The other effects are as documented: it defaults to g++ if nothing is
found.

As such, I might consider it a bug/misfeature that the EXEEXT test bombs
out - if the compiler being tested doesn't produce valid output (e.g.
because it does not exist), simply let that macro not report a result,
so that a following test (e.g. AC_PROG_CC) can determine it.