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From: "manu at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/50810] c++0x-compat does not warn about narrowing conversions
Date: Sat, 22 Oct 2011 01:25:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D50810

--- Comment #5 from Manuel L=C3=B3pez-Ib=C3=A1=C3=B1ez <manu at gcc dot gnu=
.org> 2011-10-22 01:24:25 UTC ---
(In reply to comment #4)
> (In reply to comment #3)
> > (detail: not sure about the 'if (!warn_narrowing) return;' at the begin=
ning of
> > check_narrowing: probably we don't want -Wno-narrowing to suppress the =
warnings
> > enabled of -Wc++0x-compat)
>=20
> I think what you want is that -Wc++0x-compat enables -Wnarrowing independ=
ently
> of -std=3D, but still -Wc++0x-compat -Wno-narrowing should disable it. Se=
e the
> handling of Wlonglong in c-opts.c. So:
>=20
>   if (!ok)
>     {
>       if (cxx_dialect !=3D cxx98)
>         pedwarn (input_location, OPT_Wnarrowing, "narrowing conversion of=
 %qE "
>             "from %qT to %qT inside { }", init, ftype, type);
>       else
>         warning_at (input_location, OPT_Wnarrowing, "narrowing conversion=
 of
> %qE "
>             "from %qT to %qT inside { }", init, ftype, type);
>     }
>=20

You can even make this more compact by using emit_diagnostic:

emit_diagnostic ((cxx_dialect !=3D cxx98) ? DK_PEDWARN : DK_WARNING,
                 input_location, OPT_Wnarrowing,
                 "narrowing conversion of %qE from %qT to %qT inside { }",=
=20
                  init, ftype, type);

The C FE has wrappers around something similar called pedwarn_c99, that is,=
 a
pedwarn in C99, otherwise just a warning. You could add something similar to
C++.