On Mon, 8 May 2023, 09:38 NightStrike via Gcc, <gcc@gcc.gnu.org> wrote:

> On Thu, Apr 27, 2023 at 5:41 AM Jakub Jelinek via Gcc <gcc@gcc.gnu.org>
> wrote:
> >
> > On Thu, Apr 27, 2023 at 11:35:23AM +0200, Helmut Zeisel wrote:
> > > >Von: "Jakub Jelinek" <jakub@redhat.com>
> > > >An: "Helmut Zeisel" <HZ2012@gmx.at>
> > > >Cc: gcc@gcc.gnu.org
> > > >Betreff: Re: GCC 13.1 compile error when using CXXFLAGS=-std=c++20
> > > >On Thu, Apr 27, 2023 at 11:09:19AM +0200, Helmut Zeisel via Gcc wrote:
> > > >> I compiled GCC 13.1.0 with GCC 12 and had the environment variable
> CXXFLAGS set to -std=c++20
> > > >> This gives the error (both linux and cygin)
> > > >>
> > > >> gcc-13.1.0/libstdc++-v3/src/c++98/bitmap_allocator.cc:51:23: error:
> ISO C++17 does not allow dynamic exception specifications
> > > >> 51 | _M_get(size_t __sz) throw(std::bad_alloc)
> > > >>
> > > >> After
> > > >>
> > > >> unset CXXFLAGS
> > > >>
> > > >> compilation works
> > >
> > > >Don't do it.
> > >
> > > Dont do *what*?
> > > export CXXFLAGS=... ?
> >
> > Include explicit -std= settings in such exported variables, especially
> when
> > building GCC.  Packages should decide themselves in which language
> version
> > they are written in, or if they are written in common subset, they should
> > just defer to the compiler default.
> > Note, doing that would break building even much older GCC versions.
>
> Maybe configure should catch that?
>
> diff --git a/libstdc++-v3/configure.ac b/libstdc++-v3/configure.ac
> index 0dd550a4b4b..f0d998aad94 100644
> --- a/libstdc++-v3/configure.ac
> +++ b/libstdc++-v3/configure.ac
> @@ -86,6 +86,9 @@ AH_TEMPLATE(VERSION, [Version number of package])
>  # -fno-builtin must be present here so that a non-conflicting form of
>  # std::exit can be guessed by AC_PROG_CXX, and used in later tests.
>
> +AS_CASE(["$CXXFLAGS"],
> +  [*-std=*], [AC_MSG_ERROR([don't do that])])
> +
>  save_CXXFLAGS="$CXXFLAGS"
>  CXXFLAGS="$CXXFLAGS -fno-builtin"
>  AC_PROG_CC
>

Seems like a reasonable idea at first glance, somebody could test and
submit a patch.

That pattern will match a hypothetical option like -ffoo-std=bar which is
not a -std option. I think a better pattern would be needed.