[PATCH] libstdc++: Members missing in std::numeric_limits<integer-class type>

[Jonathan Wakely]

On Wed, 2 Jul 2025 at 10:50, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On 02/07/25 03:36 +0300, Mateusz Zych wrote:
> >Hello libstdc++ Team!
> >
> >I have recently found a bug in libstdc++, that is,
> >the std::numeric_limits<> template specializations for integer-class types
> >are missing some of static data members,
> >which results in compilation errors of valid C++ code:
> >
> >   - Compiler Explorer: https://godbolt.org/z/E7z4WYfj4
> >
> >Since adding missing member constants, which are the most relevant to
> >integer-like types,
> >was not a lot of code, I have prepared a Git patch with relevant changes.
> >
> >I hope this patch is useful, Mateusz Zych
>
> Thanks, I don't think there was any reason to omit these members, and I
> agree we should add them.
>
> The patch is simple and obvious enough that I don't think we need a
> copyright assignment or DCO sign-off, so I'll push this to the
> relevant branches. Thanks!

Oh actually the radix members should be of type int, not bool. I can fix that.


>
> >From 1e83287bbd6adf6ad8f483bd2f891692e0bed0c7 Mon Sep 17 00:00:00 2001
> >From: Mateusz Zych <mte.zych@gmail.com>
> >Date: Wed, 2 Jul 2025 01:51:40 +0300
> >Subject: [PATCH] libstdc++: Added missing member constants to numeric_limits
> > specializations for integer-class types.
> >
> >25.3.4.4 Concept weakly_incrementable      [iterator.concept.winc]
> >
> >  (5) For every integer-class type I,
> >      let B(I) be a unique hypothetical extended integer type
> >      of the same signedness with the same width as I.
> >
> >      [Note 2: The corresponding
> >               hypothetical specialization numeric_limits<B(I)>
> >               meets the requirements on
> >               numeric_limits specializations for integral types.]
> >
> > (11) For every (possibly cv-qualified) integer-class type I,
> >      numeric_limits<I> is specialized such that:
> >
> >      - each static data member m
> >        has the same value as numeric_limits<B(I)>::m, and
> >
> >      - each static member function f
> >        returns I(numeric_limits<B(I)>::f()).
> >
> >libstdc++-v3/ChangeLog:
> >
> >       * include/bits/max_size_type.h
> >       (numeric_limits<__max_size_type>): New static data members.
> >       (numeric_limits<__max_diff_type>): Likewise.
> >---
> > libstdc++-v3/include/bits/max_size_type.h | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> >diff --git a/libstdc++-v3/include/bits/max_size_type.h b/libstdc++-v3/include/bits/max_size_type.h
> >index 73a6d141d5b..bc7700506a2 100644
> >--- a/libstdc++-v3/include/bits/max_size_type.h
> >+++ b/libstdc++-v3/include/bits/max_size_type.h
> >@@ -775,6 +775,9 @@ namespace ranges
> >       static constexpr bool is_signed = false;
> >       static constexpr bool is_integer = true;
> >       static constexpr bool is_exact = true;
> >+      static constexpr bool is_bounded = true;
> >+      static constexpr bool is_modulo = true;
> >+      static constexpr bool radix = 2;
> >       static constexpr int digits
> >       = __gnu_cxx::__int_traits<_Sp::__rep>::__digits + 1;
> >       static constexpr int digits10
> >@@ -802,6 +805,9 @@ namespace ranges
> >       static constexpr bool is_signed = true;
> >       static constexpr bool is_integer = true;
> >       static constexpr bool is_exact = true;
> >+      static constexpr bool is_bounded = true;
> >+      static constexpr bool is_modulo = false;
> >+      static constexpr bool radix = 2;
> >       static constexpr int digits = numeric_limits<_Sp>::digits - 1;
> >       static constexpr int digits10
> >       = static_cast<int>(digits * numbers::ln2 / numbers::ln10);
> >--
> >2.48.1
> >
>