On Tue, 7 Mar 2023, Patrick Palka wrote:
> ranges::begin() isn't guaranteed to be equality-preserving for
> non-forward ranges, so in cartesian_product_view::end we need to be
> careful about calling begin() on the first range (which could be
> non-forward) in the (non-degenerate) case where __empty_tail is false.
>
> Since we're already using a variadic lambda to compute __empty_tail, we
> might as well use that same lambda to build up the tuple of iterators
> instead of doing it via __tuple_transform.
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
>
> PR libstdc++/107572
>
> libstdc++-v3/ChangeLog:
>
> * include/std/ranges (cartesian_product_view::end): When
> building the tuple of iterators, avoid calling ranges::begin on
> the first range if __empty_tail is false.
> * testsuite/std/ranges/cartesian_product/1.cc (test07): New test.
> ---
> libstdc++-v3/include/std/ranges | 36 +++++++++++++------
> .../std/ranges/cartesian_product/1.cc | 22 ++++++++++++
> 2 files changed, 48 insertions(+), 10 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index e0cac15a64f..0de7bdef504 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -8078,26 +8078,42 @@ namespace views::__adaptor
> end() requires ((!__detail::__simple_view<_First> || ... ||
> !__detail::__simple_view<_Vs>)
> && __detail::__cartesian_product_is_common<_First, _Vs...>)
> {
> - bool __empty_tail = [this]<size_t... _Is>(index_sequence<_Is...>) {
> - return (ranges::empty(std::get<1 + _Is>(_M_bases)) || ...);
> + auto __it = [this]<size_t... _Is>(index_sequence<_Is...>) {
> + bool __empty_tail = (ranges::empty(std::get<1 + _Is>(_M_bases)) || ...);
> + auto& __first = std::get<0>(_M_bases);
> + auto __first_it = __empty_tail
> + ? ranges::begin(__first)
> + : __detail::__cartesian_common_arg_end(__first);
> + // N.B. When implementing P2165R4 this should be changed to always
> return tuple.
> + if constexpr (sizeof...(_Is) == 1)
> + return std::make_pair(std::move(__first_it),
> + ranges::begin(std::get<1 + _Is>(_M_bases))...);
> + else
> + return std::make_tuple(std::move(__first_it),
> + ranges::begin(std::get<1 + _Is>(_M_bases))...);
On second thought, it might be better to use __tuple_or_pair_t here
instead of manually determining whether to use a pair or tuple, so that
we don't forget to adjust this site when implementing P2165R4 (which
removes __tuple_or_pair_t):
-- >8 --
Subject: [PATCH] libstdc++: extraneous begin in cartesian_product_view::end
[PR107572]
ranges::begin() isn't guaranteed to be equality-preserving for
non-forward ranges, so in cartesian_product_view::end we need to avoid
calling begin() on the first range (which could be non-forward) in the
(non-degenerate) case where __empty_tail is false.
Since we're already using a variadic lambda to compute __empty_tail, we
might as well use that same lambda to build up the tuple of iterators
instead of doing it separately via __tuple_transform.
Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
PR libstdc++/107572
libstdc++-v3/ChangeLog:
* include/std/ranges (cartesian_product_view::end): When
building the tuple of iterators, avoid calling ranges::begin on
the first range if __empty_tail is false.
* testsuite/std/ranges/cartesian_product/1.cc (test07): New test.
---
libstdc++-v3/include/std/ranges | 28 ++++++++++++-------
.../std/ranges/cartesian_product/1.cc | 25 +++++++++++++++++
2 files changed, 43 insertions(+), 10 deletions(-)
diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index e0cac15a64f..d2ab79179ca 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -8078,26 +8078,34 @@ namespace views::__adaptor
end() requires ((!__detail::__simple_view<_First> || ... ||
!__detail::__simple_view<_Vs>)
&& __detail::__cartesian_product_is_common<_First, _Vs...>)
{
- bool __empty_tail = [this]<size_t... _Is>(index_sequence<_Is...>) {
- return (ranges::empty(std::get<1 + _Is>(_M_bases)) || ...);
+ auto __it = [this]<size_t... _Is>(index_sequence<_Is...>) {
+ using _Ret = __detail::__tuple_or_pair_t<iterator_t<_First>,
+ iterator_t<_Vs>...>;
+ bool __empty_tail = (ranges::empty(std::get<1 + _Is>(_M_bases)) || ...);
+ auto& __first = std::get<0>(_M_bases);
+ return _Ret{(__empty_tail
+ ? ranges::begin(__first)
+ : __detail::__cartesian_common_arg_end(__first)),
+ ranges::begin(std::get<1 + _Is>(_M_bases))...};
}(make_index_sequence<sizeof...(_Vs)>{});
- auto __it = __detail::__tuple_transform(ranges::begin, _M_bases);
- if (!__empty_tail)
- std::get<0>(__it) =
__detail::__cartesian_common_arg_end(std::get<0>(_M_bases));
return _Iterator<false>{*this, std::move(__it)};
}
constexpr _Iterator<true>
end() const requires __detail::__cartesian_product_is_common<const _First,
const _Vs...>
{
- bool __empty_tail = [this]<size_t... _Is>(index_sequence<_Is...>) {
- return (ranges::empty(std::get<1 + _Is>(_M_bases)) || ...);
+ auto __it = [this]<size_t... _Is>(index_sequence<_Is...>) {
+ using _Ret = __detail::__tuple_or_pair_t<iterator_t<const _First>,
+ iterator_t<const _Vs>...>;
+ bool __empty_tail = (ranges::empty(std::get<1 + _Is>(_M_bases)) || ...);
+ auto& __first = std::get<0>(_M_bases);
+ return _Ret{(__empty_tail
+ ? ranges::begin(__first)
+ : __detail::__cartesian_common_arg_end(__first)),
+ ranges::begin(std::get<1 + _Is>(_M_bases))...};
}(make_index_sequence<sizeof...(_Vs)>{});
- auto __it = __detail::__tuple_transform(ranges::begin, _M_bases);
- if (!__empty_tail)
- std::get<0>(__it) =
__detail::__cartesian_common_arg_end(std::get<0>(_M_bases));
return _Iterator<true>{*this, std::move(__it)};
}
diff --git a/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
b/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
index 1ec4422e6f3..8069eb0ab3c 100644
--- a/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
+++ b/libstdc++-v3/testsuite/std/ranges/cartesian_product/1.cc
@@ -3,6 +3,7 @@
#include <ranges>
#include <algorithm>
+#include <sstream>
#include <testsuite_hooks.h>
#include <testsuite_iterators.h>
@@ -178,6 +179,29 @@ test06()
return true;
}
+void
+test07()
+{
+ // PR libstdc++/107572
+ static std::istringstream ints("0 1 2 3 4");
+ struct istream_range
+ {
+ auto begin() { return std::istream_iterator<int>{ints}; }
+ auto end() { return std::istream_iterator<int>{}; }
+ using iterator_concept = std::input_iterator_tag;
+ };
+ static_assert(!ranges::forward_range<istream_range>
+ && ranges::common_range<istream_range>);
+ istream_range r;
+ int i = 0;
+ for (auto [v] : views::cartesian_product(r))
+ {
+ VERIFY( v == i );
+ ++i;
+ };
+ VERIFY( i == 5 );
+}
+
int
main()
{
@@ -187,4 +211,5 @@ main()
test04();
test05();
static_assert(test06());
+ test07();
}
--
2.40.0.rc0.57.g454dfcbddf
