[PATCH] Fix compilation of .c files as C++ when using Clang

[Pedro Alves]

On 9/16/20 4:49 PM, Gary Benson via Gdb-patches wrote:
> Hi all,
> 
> In commit 221db974e653659edb280787af1b3efdd1615083, this patch:
> 
>     2020-06-24  Pedro Alves  <palves@redhat.com>
> 
>     * lib/gdb.exp (gdb_compile): Pass "-x c++" explicitly when
>     compiling C++ programs.
> 
> attempted to fix problems with testcases that compile .c files
> with the C++ compiler.  They pass the "c++" option to gdb_compile,
> resulting in the following error when using Clang:
> 
>     gdb compile failed, clang-10: warning: treating 'c' input as 'c++'
>     when in C++ mode, this behavior is deprecated [-Wdeprecated]
> 
> This fix did not work for gdb.base/infcall-nested-structs-c++.exp,
> however: the "-x c++" appeared in the compiler's commandline after
> the .c file, so the option was not enabled for that file.  

Huh.  You're missing explaining why this testcase is different.

So I digged, and noticed that the original testcases my patch
was addressing where using build_executable/prepare_for_testing.
They all end up in build_executable_from_specs, which compiles and links
in separate steps, using gdb_compile -- the compile step passes $type=object
to gdb_compile.  While the link step passes $type=executable.

Now, gdb.base/infcall-nested-structs-c++.exp uses gdb_compile directly
instead, and it passes $type=executable to compile and link all in one step.

So the difference is that when $type=object, the sources appear at the
end, and when $type=executable, the sources are at the beginning.

Odd.

Digging a little deeper, I found it in dejagnu's sources,
in default_target_compile:

    # This is obscure: we put SOURCES at the end when building an
    # object, because otherwise, in some situations, libtool will
    # become confused about the name of the actual source file.
    if {$type == "object"} {
	set opts "$add_flags $sources"
    } else {
	set opts "$sources $add_flags"
    }

There you go...

> This
> commit moves the "-x c++" earlier in the compiler's commandline.
> Unfortunately this then broke the testcase that required the original
> fix, gdb.compile/compile-cplus.exp: the "-x c++" was being parsed
> for the linker pass, causing the compiler to attempt to parse the
> .o files as C++.  This commit makes passing "-x c++" conditional on
> the source being a .c file.
> 
> Checked on Fedora 32 x86_64, GCC and clang.  Is this ok to commit?

OK.  But I'd appreciate it if you included some of the info above
in the commit log.

Thanks,
Pedro Alves