[PATCH] libsdc++: Enable asserts in repeat_view::_Iterator uncoditionally.

[Jonathan Wakely]

On Fri, 18 Sept 2026 at 15:50, Tomasz Kamiński <tkaminsk@redhat.com> wrote:
>
> The checks against _M_current being negive were disabled for cases where
> _Bound denotes unreachable_sentinel_t. However, _M_current has ptrdiff_t
> type in that case, and this check are as meaningfull are for integer bound.

The standard seems to suggest that it's OK to pass a negative bound
for an unreachable sentinel.

Preconditions: If Bound is not unreachable_sentinel_t, b ≥ 0.

This change would cause assertions for the code that isn't forbidden
by the standard. Or am I missing something?


>
> libstdc++-v3/ChangeLog:
>
>         * include/std/ranges (repeat_view::_Iterator::_Iterator)
>         (repeat_view::_Iterator::operator--)
>         (repeat_view::_Iterator::operator+=)
>         (repeat_view::_Iterator::operator-=): Unconditionally assert
>         on _M_current having non-negative value.
> ---
> Noticed that when working on extracting _Repeat_iterator (that I
> templated with _IndexType). Worth to be separate.
>
> Testing on x86_64-linux. *repeat* view passed with all standard
> modes, debug and assertions. OK for trunk?
>
>  libstdc++-v3/include/std/ranges | 14 ++++----------
>  1 file changed, 4 insertions(+), 10 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index 1c0991cae13..a0ce157d4f2 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -8075,10 +8075,7 @@ namespace views::__adaptor
>      constexpr explicit
>      _Iterator(const _Tp* __value, __index_type __bound = __index_type())
>      : _M_value(__value), _M_current(__bound)
> -    {
> -      if constexpr (!same_as<_Bound, unreachable_sentinel_t>)
> -       __glibcxx_assert(__bound >= 0);
> -    }
> +    { __glibcxx_assert(__bound >= 0); }
>
>      friend repeat_view;
>
> @@ -8112,8 +8109,7 @@ namespace views::__adaptor
>      constexpr _Iterator&
>      operator--()
>      {
> -      if constexpr (!same_as<_Bound, unreachable_sentinel_t>)
> -       __glibcxx_assert(_M_current > 0);
> +      __glibcxx_assert(_M_current > 0);
>        --_M_current;
>        return *this;
>      }
> @@ -8129,8 +8125,7 @@ namespace views::__adaptor
>      constexpr _Iterator&
>      operator+=(difference_type __n)
>      {
> -      if constexpr (!same_as<_Bound, unreachable_sentinel_t>)
> -       __glibcxx_assert(_M_current + __n >= 0);
> +      __glibcxx_assert(_M_current + __n >= 0);
>        _M_current += __n;
>        return *this;
>      }
> @@ -8138,8 +8133,7 @@ namespace views::__adaptor
>      constexpr _Iterator&
>      operator-=(difference_type __n)
>      {
> -      if constexpr (!same_as<_Bound, unreachable_sentinel_t>)
> -       __glibcxx_assert(_M_current - __n >= 0);
> +      __glibcxx_assert(_M_current - __n >= 0);
>        _M_current -= __n;
>        return *this;
>      }
> --
> 2.55.0
>