Re: [PATCH] libstdc++: Set active union member in constexpr std::string [PR103295]

[Patrick Palka <ppalka@redhat.com>]

On Fri, 11 Nov 2022, Jonathan Wakely via Libstdc++ wrote:

> On Fri, 11 Nov 2022 at 11:23, Nathaniel Shead via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > Hi,
> >
> > Below is a patch to fix std::string in constexpr contexts on Clang. This
> > was originally fixed in the commits attached to PR103295, but a later
> > commit 98a0d72a seems to have mistakenly undone this.
> >
> > Tested on x86_64-linux. Verified using clang-14 and clang-15 that the
> > fix works. I haven't added anything to the test suite, since this issue
> > is only detected by clang.
> >
> > This is my first time contributing, so please let me know if I've done
> > anything wrong or missed something. Thanks!
> 
> Thanks for the patch, I'll get this committed today.
> 
> The only thing I had to fix was the indentation in the commit log. The
> second line of the ChangeLog should be aligned with the * not the text
> following it (so indented by a single tab).
> 
> 
> >
> > Nathaniel
> >
> > -- >8 --
> >
> > Clang still complains about using std::string in constexpr contexts due
> > to the changes made in commit 98a0d72a. This patch ensures that we set
> > the active member of the union as according to [class.union.general] p6.
> >
> > libstdc++-v3/ChangeLog:
> >
> >         PR libstdc++/103295
> >         * include/bits/basic_string.h (_M_use_local_data): Set active
> >           member to _M_local_buf.
> >
> > Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> > ---
> >  libstdc++-v3/include/bits/basic_string.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> > index 9c2b57f5a1d..2790fd49b05 100644
> > --- a/libstdc++-v3/include/bits/basic_string.h
> > +++ b/libstdc++-v3/include/bits/basic_string.h
> > @@ -352,8 +352,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> >        {
> >  #if __cpp_lib_is_constant_evaluated
> >         if (std::is_constant_evaluated())
> > -         for (_CharT& __c : _M_local_buf)
> > -           __c = _CharT();
> > +         for (size_type i = 0; i <= _S_local_capacity; ++i)
> > +           _M_local_buf[i] = _CharT();

Just a minor nit, but we should probably uglify i to __i here.

> >  #endif
> >         return _M_local_data();
> >        }
> > --
> > 2.34.1
> >
> 
>