[PATCH] libstdc++: Support constexpr formatting for vector<bool>::reference.

[Jonathan Wakely]

On Thu, 6 Aug 2026 at 12:08, Tomasz Kamiński <tkaminsk@redhat.com> wrote:

> This specialization was made constexpr as part of P3391R2, "constexpr
> format", and was missed in r17-1162-g42f9bf2e3c56ae.
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/vector (formatter<_Bit_reference, _CharT>::format)
>         [__glibcxx_constexpr_format]: Declare as constexpr.
>         * testsuite/23_containers/vector/bool/format.cc
>         [__glibcxx_constexpr_format]: Run test suite at compile
>         time.
> ---
> Noted this when I expanded constexpr queue/priority_queue to
> make their formatters constexpr here:
> https://gcc.gnu.org/pipermail/libstdc++/2026-August/067451.html
>
> Testing on x86_64-linux. Modified test passed with all standard
> modes. OK for trunk when all test passes?
>

OK



>
>  libstdc++-v3/include/std/vector               |  3 +++
>  .../23_containers/vector/bool/format.cc       | 26 ++++++++++++++++---
>  2 files changed, 25 insertions(+), 4 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/vector
> b/libstdc++-v3/include/std/vector
> index 86e42b22ea5..aeb7edce710 100644
> --- a/libstdc++-v3/include/std/vector
> +++ b/libstdc++-v3/include/std/vector
> @@ -148,6 +148,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        // Standard declares this as template accepting unconstrained
>        // FormatContext type.
>        template<typename _Out>
> +#ifdef __glibcxx_constexpr_format // C++ >= 26 && HOSTED && CXX11 string
> +       constexpr
> +#endif
>         typename basic_format_context<_Out, _CharT>::iterator
>         format(const _GLIBCXX_STD_C::_Bit_reference& __u,
>                basic_format_context<_Out, _CharT>& __fc) const
> diff --git a/libstdc++-v3/testsuite/23_containers/vector/bool/format.cc
> b/libstdc++-v3/testsuite/23_containers/vector/bool/format.cc
> index 833727f4b41..856687c49f6 100644
> --- a/libstdc++-v3/testsuite/23_containers/vector/bool/format.cc
> +++ b/libstdc++-v3/testsuite/23_containers/vector/bool/format.cc
> @@ -5,12 +5,18 @@
>  #include <vector>
>  #include <testsuite_hooks.h>
>
> +#ifdef __glibcxx_constexpr_format
> +# define CONSTEXPR constexpr
> +#else
> +# define CONSTEXPR
> +#endif
> +
>  static_assert(!std::formattable<std::vector<bool>::reference, int>);
>  static_assert(!std::formattable<std::vector<bool>::reference, char32_t>);
>
>  static_assert(std::enable_nonlocking_formatter_optimization<std::vector<bool>::reference>);
>
>  template<typename... Args>
> -bool
> +CONSTEXPR bool
>  is_format_string_for(const char* str, Args&&... args)
>  {
>    try {
> @@ -24,7 +30,7 @@ is_format_string_for(const char* str, Args&&... args)
>  #define WIDEN_(C, S) ::std::__format::_Widen<C>(S, L##S)
>  #define WIDEN(S) WIDEN_(CharT, S)
>
> -void
> +CONSTEXPR void
>  test_format_string()
>  {
>    std::vector<bool> v(1, true);
> @@ -36,7 +42,7 @@ test_format_string()
>  }
>
>  template<typename CharT>
> -void
> +CONSTEXPR void
>  test_output()
>  {
>    std::basic_string<CharT> res;
> @@ -65,9 +71,21 @@ test_output()
>    VERIFY( res == WIDEN("[1, 0]") );
>  }
>
> -int main()
> +CONSTEXPR bool
> +test_all()
>  {
>    test_format_string();
>    test_output<char>();
>    test_output<wchar_t>();
> +
> +  return true;
> +}
> +
> +#ifdef __glibcxx_constexpr_format
> +static_assert(test_all());
> +#endif
> +
> +int main()
> +{
> +  test_all();
>  }
> --
> 2.55.0
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://gcc.gnu.org/pipermail/libstdc++/attachments/20260817/2971ac5e/attachment-0001.htm>