How to query GCC warnings for C++? - gcc

Question

GCC allows querying available warning flags specific for C++ language with the syntax:
g++ -Q --help=warning,c++
Adding warning flags to the call includes them in the result:
g++ -Wall -Q --help=warning,c++
However, it seems the call is done from the C point of view and I don't know how to do it from the C++ point of view. If the call includes C++-only warning, like:
g++ -Wnon-virtual-dtor -Q --help=warning,c++
the output contains a message:
cc1: warning: command line option ‘-Wnon-virtual-dtor’ is valid for C++/ObjC++ but not for C
and still shows the warning as disabled:
-Wnon-virtual-dtor [disabled]
Note, that this happens regardless of whether the call is done using g++ or gcc.
The same with C-only -Wbad-function-cast behaves in an expected way:
gcc -Wbad-function-cast -Q --help=warning,c
There is no extra message and reported warning status changes between [disabled] and [enabled]. Again, regardless of whether g++ or gcc is used.
I'm using GCC version 7.3.0. Although the issue seems to apply to many if not all versions. It can be observed through Compiler Explorer.
So, is there a way to do this query with respect to given language?

Answer 1

Yes, your observations are correct.
Probably this is not the intended behavior, and if you care about this feature, then I suggest reporting it upstream.
Note that this works, however:
touch 1.cc
g++ -Wnon-virtual-dtor -Q --help=warning,c++ 1.cc
I.e. if there's an input file with a proper extension, then the correct compiler proper executable is invoked: cc1plus, not cc1. The latter is the default if no input files are present. I did some quick debugging, and here's how that happens:
// gcc.c:
driver::do_spec_on_infiles () const
{
...
for (i = 0; (int) i < n_infiles; i++)
{
...
/* Figure out which compiler from the file's suffix. */
input_file_compiler
= lookup_compiler (infiles[i].name, input_filename_length,
infiles[i].language);
if (input_file_compiler)
{
...
value = do_spec (input_file_compiler->spec);
And input_file_compiler at that point is the C compiler, because
p n_infiles
$9 = 1
(gdb) p infiles[0]
$10 = {name = 0x4cbfb0 "help-dummy", language = 0x4cbfae "c", incompiler = 0x58a920, compiled = false, preprocessed = false}
Here's how the dummy file got created (function process_command in the same file):
if (n_infiles == 0
&& (print_subprocess_help || print_help_list || print_version))
{
/* Create a dummy input file, so that we can pass
the help option on to the various sub-processes. */
add_infile ("help-dummy", "c");
}