Re: [PATCH] libstdc++: invalid default init in _CachedPosition [PR101231]

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Tue, 13 Jul 2021 at 20:09, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> The primary template for _CachedPosition is a dummy implementation for
> non-forward ranges, the iterators for which generally can't be cached.
> Because this implementation doesn't actually cache anything, _M_has_value
> is defined to be false and so calls to _M_get (which are always guarded
> by _M_has_value) are unreachable.
>
> Still, to suppress a "control reaches end of non-void function" warning
> I made _M_get return {}, but after P2325 input iterators are no longer
> necessarily default constructible so this workaround now breaks valid
> programs.
>
> This patch fixes this by instead using __builtin_unreachable to squelch
> the warning.
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

Yes, thanks.


>
>         PR libstdc++/101231
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/ranges (_CachedPosition::_M_get): For non-forward
>         ranges, just call __builtin_unreachable.
>         * testsuite/std/ranges/istream_view.cc (test05): New test.
> ---
>  libstdc++-v3/include/std/ranges                   |  2 +-
>  libstdc++-v3/testsuite/std/ranges/istream_view.cc | 12 ++++++++++++
>  2 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index df74ac9dc19..d791e15d096 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -1232,7 +1232,7 @@ namespace views::__adaptor
>         _M_get(const _Range&) const
>         {
>           __glibcxx_assert(false);
> -         return {};
> +         __builtin_unreachable();
>         }
>
>         constexpr void
> diff --git a/libstdc++-v3/testsuite/std/ranges/istream_view.cc b/libstdc++-v3/testsuite/std/ranges/istream_view.cc
> index 369790e89e5..2f15f787250 100644
> --- a/libstdc++-v3/testsuite/std/ranges/istream_view.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/istream_view.cc
> @@ -83,6 +83,17 @@ test04()
>    static_assert(!std::forward_iterator<It>);
>  }
>
> +void
> +test05()
> +{
> +  // PR libstdc++/101231
> +  auto words = std::istringstream{"42"};
> +  auto is = ranges::istream_view<int>(words);
> +  auto r = is | views::filter([](auto) { return true; });
> +  for (auto x : r)
> +    ;
> +}
> +
>  int
>  main()
>  {
> @@ -90,4 +101,5 @@ main()
>    test02();
>    test03();
>    test04();
> +  test05();
>  }
> --
> 2.32.0.170.gd486ca60a5
>