On Fri, Feb 13, 2026 at 09:34:56PM +0100, Jakub Jelinek wrote:
> On Fri, Feb 13, 2026 at 03:17:53PM -0500, Marek Polacek wrote:
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> >
> > -- >8 --
> > [bit.cast]/2: Mandates: Neither To nor From are consteval-only types
> >
> > but we are not checking this, so the attached test compiled.
> >
> > PR libstdc++/124096
> >
> > libstdc++-v3/ChangeLog:
> >
> > * include/std/bit: Check that neither To nor From are consteval-only
> > types.
> > * testsuite/26_numerics/bit/bit.cast/124096.cc: New test.
> > ---
> > libstdc++-v3/include/std/bit | 4 ++++
> > .../testsuite/26_numerics/bit/bit.cast/124096.cc | 13 +++++++++++++
> > 2 files changed, 17 insertions(+)
> > create mode 100644
> > libstdc++-v3/testsuite/26_numerics/bit/bit.cast/124096.cc
> >
> > diff --git a/libstdc++-v3/include/std/bit b/libstdc++-v3/include/std/bit
> > index 6ea0f6eef83..f33acb999cf 100644
> > --- a/libstdc++-v3/include/std/bit
> > +++ b/libstdc++-v3/include/std/bit
> > @@ -93,6 +93,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > && is_trivially_copyable_v<_To> && is_trivially_copyable_v<_From>
> > #endif
> > {
> > +#if __cpp_impl_reflection >= 202506L
> > + static_assert(!is_consteval_only_v<_To> &&
> > !is_consteval_only_v<_From>,
> > + "neither To nor From can be consteval-only types");
> > +#endif
> > return __builtin_bit_cast(_To, __from);
> > }
> > #endif // __cpp_lib_bit_cast
> > diff --git a/libstdc++-v3/testsuite/26_numerics/bit/bit.cast/124096.cc
> > b/libstdc++-v3/testsuite/26_numerics/bit/bit.cast/124096.cc
> > new file mode 100644
> > index 00000000000..12c93f712df
> > --- /dev/null
> > +++ b/libstdc++-v3/testsuite/26_numerics/bit/bit.cast/124096.cc
> > @@ -0,0 +1,13 @@
> > +// PR libstdc++/124096
> > +// { dg-do compile { target c++26 } }
> > +// { dg-additional-options "-freflection" }
> > +
> > +#include <bit>
> > +
> > +consteval void
> > +f ()
> > +{
> > + (void) std::bit_cast<long long>(^^int); // { dg-error "here" }
> > +}
>
> Shouldn't the testcase check both From and To separately?
> Also, wouldn't it be better to use say
> auto a = ^^int;
> auto *b = &a;
> void *c = nullptr;
> std::bit_cast<void*>(b);
> std::bit_cast<decltype (^^int)*>(c);
> ? What sizeof(^^int) is is implementation-defined (I think
> in gcc we use sizeof(void*) aka sizeof(__INTPTR_TYPE__) or so,
> so long long can error differently e.g. on ia32, in clang it is
> 16 regardless of -m32/-m64), while pointer to decltype(^^int) is
> also consteval-only but better should be the same as other pointers.
Thanks, I've adjusted the testcase as you're proposing.
Marek
