[PATCH] libstdc++: Mandate power of two alignment for std::is_sufficiently_aligned.

[Tomasz Kaminski]

On Wed, Apr 22, 2026 at 1:46 PM Jonathan Wakely <jwakely@redhat.com> wrote:

> On Wed, 22 Apr 2026 at 12:39, Tomasz Kamiński <tkaminsk@redhat.com> wrote:
> >
> > This implements LWG4290. Missing Mandates clauses on
> is_sufficiently_aligned.
> >
> > libstdc++-v3/ChangeLog:
> >
> >         * include/bits/align.h (is_sufficiently_aligned): Mandate
> >         that _Align is power of two.
> >         * testsuite/std/time/format/simple.cc: New test.
>
> Did you 'git add' the wrong test?
>
Yes, will post v2 soon.

>
> > ---
> > No adding resolve issue comment, as this is C++26 feature.
> >
> > Tested on x86_64-linux. OK for trunk?
> >
> >  libstdc++-v3/include/bits/align.h                |  5 ++++-
> >  libstdc++-v3/testsuite/std/time/format/simple.cc | 11 +++++++++++
> >  2 files changed, 15 insertions(+), 1 deletion(-)
> >  create mode 100644 libstdc++-v3/testsuite/std/time/format/simple.cc
> >
> > diff --git a/libstdc++-v3/include/bits/align.h
> b/libstdc++-v3/include/bits/align.h
> > index 1a14e282216..3c8355d0e07 100644
> > --- a/libstdc++-v3/include/bits/align.h
> > +++ b/libstdc++-v3/include/bits/align.h
> > @@ -116,7 +116,10 @@ align(size_t __align, size_t __size, void*& __ptr,
> size_t& __space) noexcept
> >      [[nodiscard,__gnu__::__always_inline__]]
> >      inline bool
> >      is_sufficiently_aligned(_Tp* __ptr)
> > -    { return reinterpret_cast<__UINTPTR_TYPE__>(__ptr) % _Align == 0; }
> > +    {
> > +      static_assert(std::has_single_bit(_Align));
> > +      return reinterpret_cast<__UINTPTR_TYPE__>(__ptr) % _Align == 0;
> > +    }
> >  #endif // __glibcxx_is_sufficiently_aligned
> >
> >  _GLIBCXX_END_NAMESPACE_VERSION
> > diff --git a/libstdc++-v3/testsuite/std/time/format/simple.cc
> b/libstdc++-v3/testsuite/std/time/format/simple.cc
> > new file mode 100644
> > index 00000000000..db25a460bf7
> > --- /dev/null
> > +++ b/libstdc++-v3/testsuite/std/time/format/simple.cc
> > @@ -0,0 +1,11 @@
> > +// { dg-do run { target c++20 } }
> > +// { dg-timeout-factor 2 }
> > +
> > +#include <chrono>
> > +#include <vector>
> > +#include <testsuite_hooks.h>
> > +
> > +int main()
> > +{
> > +  auto s = std::format("{:%F %T}", std::chrono::system_clock::now());
> > +}
> > --
> > 2.53.0
> >
>
>
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