Re: [PATCH 1/2] libstdc++: Implement ranges::adjacent_view from P2321R2

[Jonathan Wakely <jwakely@redhat.com>]

On Tue, 30 Aug 2022 at 18:14, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?


> +    constexpr
> +    _Iterator(__as_sentinel, iterator_t<_Base> __first, iterator_t<_Base> __last)
> +    {
> +      if constexpr (!bidirectional_range<_Base>)
> +       for (auto& __it : _M_current)
> +         __it = __last;
> +      else
> +       for (ssize_t __i = _Nm-1; __i >= 0; --__i)

ssize_t is defined by POSIX in <sys/types.h> but isn't in ISO C or
C++. It looks like MinGW defines it to ... something ... sometimes,
but I don't think we can rely on it for non-POSIX targets.


> +    template<size_t _Nm>
> +      struct _Adjacent : __adaptor::_RangeAdaptorClosure
> +      {
> +       template<viewable_range _Range>
> +         requires (_Nm == 0) || __detail::__can_adjacent_view<_Nm, _Range>
> +         [[nodiscard]]
> +         constexpr auto
> +         operator()(_Range&& __r) const

Does this attribute actually work here?

I thought we needed to use `operator() [[nodiscard]]` for functions
with a requires-clause, because otherwise the attribute gives a parse
error. Maybe I've misremembered the problem, and it just doesn't give
a -Wunused-result warning. The decl above is setting off my spidey
sense for some reason though.