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[libc++][ranges] LWG3715: view_interface::empty is overconstrained #85004

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Merged
merged 6 commits into from
Mar 20, 2024
Merged

[libc++][ranges] LWG3715: view_interface::empty is overconstrained #85004

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7a0c2c5
[libc++][ranges] implement LWG3715
Mar 13, 2024
29e95e1
[libc++][ranges] implement 'lwg3715.pass.cpp'
Mar 13, 2024
a34519f
[libc++][ranges] remove 'lwg3715.pass.cpp'
Mar 13, 2024
56e2ee5
[libc++][ranges] format code with 'clang-format'
Mar 13, 2024
e63525b
[libc++][ranges] add missing headers to 'view_interface.h'
Mar 13, 2024
a290ef5
[libc++][ranges] add missing test cases to 'view.interface.pass.cpp'
Mar 14, 2024
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2 changes: 1 addition & 1 deletion libcxx/docs/Status/Cxx23Issues.csv
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Original file line number Diff line number Diff line change
Expand Up @@ -181,7 +181,7 @@
"`3711 <https://wg21.link/LWG3711>`__","Missing preconditions for slide_view constructor","July 2022","","","|ranges|"
"`3712 <https://wg21.link/LWG3712>`__","``chunk_view`` and ``slide_view`` should not be ``default_initializable``","July 2022","","","|ranges|"
"`3713 <https://wg21.link/LWG3713>`__","Sorted with respect to comparator (only)","July 2022","",""
"`3715 <https://wg21.link/LWG3715>`__","``view_interface::empty`` is overconstrained","July 2022","","","|ranges|"
"`3715 <https://wg21.link/LWG3715>`__","``view_interface::empty`` is overconstrained","July 2022","|Complete|","19.0","|ranges|"
"`3719 <https://wg21.link/LWG3719>`__","Directory iterators should be usable with default sentinel","July 2022","|Complete|","17.0","|ranges|"
"`3721 <https://wg21.link/LWG3721>`__","Allow an ``arg-id`` with a value of zero for ``width`` in ``std-format-spec``","July 2022","|Complete|","16.0","|format|"
"`3724 <https://wg21.link/LWG3724>`__","``decay-copy`` should be constrained","July 2022","|Complete|","14.0"
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17 changes: 13 additions & 4 deletions libcxx/include/__ranges/view_interface.h
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Expand Up @@ -21,6 +21,7 @@
#include <__ranges/access.h>
#include <__ranges/concepts.h>
#include <__ranges/empty.h>
#include <__ranges/size.h>
#include <__type_traits/is_class.h>
#include <__type_traits/make_unsigned.h>
#include <__type_traits/remove_cv.h>
Expand Down Expand Up @@ -51,16 +52,24 @@ class view_interface {
public:
template <class _D2 = _Derived>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool empty()
Comment on lines 53 to 54
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@xiaoyang-sde xiaoyang-sde Mar 13, 2024 •
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Although unrelated to this pull request, I'm curious about the reason behind this method being a member template. It appears that this differs from the implementation in libstdc++.

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This could be related to a Clang bug when we implemented this early on. I vaguely remember that there were bugs related to constraints on member functions in Clang a while back, and I think we had to work around it in a few places. But it could also be my memory playing tricks on me, so it's worth double-checking that information.

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This could be related to a Clang bug when we implemented this early on. I vaguely remember that there were bugs related to constraints on member functions in Clang a while back, and I think we had to work around it in a few places. But it could also be my memory playing tricks on me, so it's worth double-checking that information.

Ah I see. Thanks for the explanation.

requires forward_range<_D2>
requires sized_range<_D2> || forward_range<_D2>
{
return ranges::begin(__derived()) == ranges::end(__derived());
if constexpr (sized_range<_D2>) {
return ranges::size(__derived()) == 0;
} else {
return ranges::begin(__derived()) == ranges::end(__derived());
}
}

template <class _D2 = _Derived>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool empty() const
requires forward_range<const _D2>
requires sized_range<const _D2> || forward_range<const _D2>
{
return ranges::begin(__derived()) == ranges::end(__derived());
if constexpr (sized_range<const _D2>) {
return ranges::size(__derived()) == 0;
} else {
return ranges::begin(__derived()) == ranges::end(__derived());
}
}

template <class _D2 = _Derived>
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21 changes: 21 additions & 0 deletions libcxx/test/std/ranges/range.utility/view.interface/view.interface.pass.cpp
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Expand Up @@ -39,6 +39,14 @@ struct InputRange : std::ranges::view_interface<InputRange> {
constexpr InputIter end() const { return InputIter(buff + 8); }
};

struct SizedInputRange : std::ranges::view_interface<SizedInputRange> {
int buff[8] = {0, 1, 2, 3, 4, 5, 6, 7};
constexpr InputIter begin() const { return InputIter(buff); }
constexpr sentinel_wrapper<InputIter> end() const { return sentinel_wrapper(InputIter(buff + 8)); }
constexpr std::size_t size() const { return 8; }
};
static_assert(std::ranges::sized_range<SizedInputRange>);

struct NotSizedSentinel {
using value_type = int;
using difference_type = std::ptrdiff_t;
Expand Down Expand Up @@ -155,11 +163,24 @@ concept BoolOpInvocable = requires (T const& obj) { bool(obj); };

constexpr bool testEmpty() {
static_assert(!EmptyInvocable<InputRange>);
// LWG 3715: `view_interface::empty` is overconstrained
static_assert(EmptyInvocable<SizedInputRange>);
static_assert( EmptyInvocable<ForwardRange>);

static_assert(!BoolOpInvocable<InputRange>);
static_assert(BoolOpInvocable<SizedInputRange>);
static_assert( BoolOpInvocable<ForwardRange>);

SizedInputRange sizedInputRange;
assert(!sizedInputRange.empty());
assert(!static_cast<SizedInputRange const&>(sizedInputRange).empty());

assert(sizedInputRange);
assert(static_cast<SizedInputRange const&>(sizedInputRange));

assert(!std::ranges::empty(sizedInputRange));
assert(!std::ranges::empty(static_cast<SizedInputRange const&>(sizedInputRange)));

ForwardRange forwardRange;
assert(!forwardRange.empty());
assert(!static_cast<ForwardRange const&>(forwardRange).empty());
Expand Down