On Thu, 10 Feb 2022 at 16:58, Patrick Palka via Libstdc++
<libstd...@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.
OK
>
> PR libstdc++/103904
> 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.
>
> (cherry picked from commit 1af937eb6246ad7f63ebff03590e9eede33aca81)
> ---
> 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 bf6cfae2a6e..a4228ba9aa0 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -1221,7 +1221,7 @@ 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 af76a1ab39e..f5c0c2a6bb0 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)
> + ;
> +}
> +
> void
> test06()
> {
> @@ -99,5 +110,6 @@ main()
> test02();
> test03();
> test04();
> + test05();
> test06();
> }
> --
> 2.35.1.102.g2b9c120970
>
