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subrange get<0> requires a copyable iterator #2425

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Jan 22, 2022
Merged

subrange get<0> requires a copyable iterator #2425

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1659202
subrange `get<0>` requires a copyable iterator
CaseyCarter Dec 17, 2021
a570133
Implement LWG-3589 _correctly_
CaseyCarter Dec 18, 2021
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2 changes: 1 addition & 1 deletion stl/inc/xutility
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Original file line number Diff line number Diff line change
Expand Up @@ -3487,7 +3487,7 @@ namespace ranges {

// clang-format off
template <size_t _Idx, class _It, class _Se, subrange_kind _Ki>
requires (_Idx < 2)
requires ((_Idx == 0 && copyable<_It>) || _Idx == 1)
_NODISCARD constexpr auto get(const subrange<_It, _Se, _Ki>& _Val) {
if constexpr (_Idx == 0) {
return _Val.begin();
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44 changes: 43 additions & 1 deletion tests/std/tests/P0896R4_ranges_subrange/test.compile.pass.cpp
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Expand Up @@ -4,8 +4,8 @@
// Covers ranges::view_interface and ranges::subrange

#include <cassert>
#include <concepts>
#include <forward_list>
#include <istream>
#include <list>
#include <ranges>
#include <string_view>
Expand All @@ -24,6 +24,7 @@ using std::output_iterator_tag, std::input_iterator_tag, std::forward_iterator_t
int main() {} // COMPILE-ONLY

void test_LWG_3470() {
// LWG-3470 relaxed the "convertible-to-non-slicing" requirements to allow this non-slicing case
int a[] = {1, 2, 3};
int* b[] = {&a[2], &a[0], &a[1]};
[[maybe_unused]] auto c = std::ranges::subrange<const int* const*>(b);
Expand Down Expand Up @@ -1483,3 +1484,44 @@ namespace test_subrange {
STATIC_ASSERT(test_tuple<subrange<int*, std::unreachable_sentinel_t, subrange_kind::sized>>());
STATIC_ASSERT(test_tuple<subrange<int*, std::unreachable_sentinel_t, subrange_kind::unsized>>());
} // namespace test_subrange

namespace test_lwg_3589 {
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"LWG" is capitalized on 26.

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I can live with this, no need to reset testing.

// LWG-3589 added a Constraint to std::get<0>(const subrange&) to require the iterator type to be copyable
template <class T, size_t I>
concept CanGet = requires {
std::get<I>(std::declval<T>());
};

template <class T, size_t I>
concept CanRangesGet = requires {
ranges::get<I>(std::declval<T>());
};

template <class I, class S>
constexpr bool test() {
using ranges::subrange;

STATIC_ASSERT(std::input_iterator<I>);
STATIC_ASSERT(std::sentinel_for<S, I>);

STATIC_ASSERT(CanGet<const subrange<I, S>&, 0> == std::copyable<I>);
STATIC_ASSERT(CanGet<const subrange<I, S>&, 1>);
STATIC_ASSERT(!CanGet<const subrange<I, S>&, 2>);
STATIC_ASSERT(CanGet<subrange<I, S>, 0>);
STATIC_ASSERT(CanGet<subrange<I, S>, 1>);
STATIC_ASSERT(!CanGet<subrange<I, S>, 2>);

STATIC_ASSERT(CanRangesGet<const subrange<I, S>&, 0> == std::copyable<I>);
STATIC_ASSERT(CanRangesGet<const subrange<I, S>&, 1>);
STATIC_ASSERT(!CanRangesGet<const subrange<I, S>&, 2>);
STATIC_ASSERT(CanRangesGet<subrange<I, S>, 0>);
STATIC_ASSERT(CanRangesGet<subrange<I, S>, 1>);
STATIC_ASSERT(!CanRangesGet<subrange<I, S>, 2>);

return true;
}

// Validate with a copyable iterator type, and with a move-only iterator type
STATIC_ASSERT(test<int*, int*>());
STATIC_ASSERT(test<ranges::iterator_t<ranges::istream_view<int>>, ranges::sentinel_t<ranges::istream_view<int>>>());
} // namespace test_lwg_3589