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Implement LWG-4083 views::as_rvalue should reject non-input ranges #4786

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merged 5 commits into from
Jul 11, 2024
Merged

Implement LWG-4083 views::as_rvalue should reject non-input ranges #4786

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898428a
Implement LWG-4083
frederick-vs-ja Jul 4, 2024
e89a1ff
Workaround for DevCom-10698021
frederick-vs-ja Jul 6, 2024
7ce567e
Merge branch 'main' into lwg-4083
frederick-vs-ja Jul 6, 2024
5450af7
"perma-workaround" the direct-initialization
frederick-vs-ja Jul 7, 2024
7a66606
Code review feedback.
StephanTLavavej Jul 8, 2024
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6 changes: 4 additions & 2 deletions stl/inc/ranges
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Original file line number Diff line number Diff line change
Expand Up @@ -1523,16 +1523,18 @@ namespace ranges {
constexpr auto _Compile_time_max_size<const as_rvalue_view<_Rng>> = _Compile_time_max_size<const _Rng>;

namespace views {
// direct-intialization is specified in the Standard (N4981 [range.as.rvalue.overview]/2.2)
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// and needed for EDG, DevCom-10698021
template <class _Rng>
concept _Can_as_rvalue = requires(_Rng&& __r) { as_rvalue_view{static_cast<_Rng&&>(__r)}; };
concept _Can_as_rvalue = requires(_Rng&& __r) { as_rvalue_view(static_cast<_Rng&&>(__r)); };

class _As_rvalue_fn : public _Pipe::_Base<_As_rvalue_fn> {
private:
enum class _St { _None, _All, _As_rvalue };

template <class _Rng>
_NODISCARD static consteval _Choice_t<_St> _Choose() noexcept {
if constexpr (same_as<range_rvalue_reference_t<_Rng>, range_reference_t<_Rng>>) {
if constexpr (input_range<_Rng> && same_as<range_rvalue_reference_t<_Rng>, range_reference_t<_Rng>>) {
return {_St::_All, noexcept(views::all(_STD declval<_Rng>()))};
} else if constexpr (_Can_as_rvalue<_Rng>) {
return {_St::_As_rvalue, noexcept(as_rvalue_view{_STD declval<_Rng>()})};
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11 changes: 11 additions & 0 deletions tests/std/tests/P2446R2_views_as_rvalue/test.cpp
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Expand Up @@ -401,6 +401,17 @@ void test_example_from_p2446r2() {
assert(ranges::all_of(words, ranges::empty)); // all strings from words are empty (implementation assumption)
}

// LWG-4083 "views::as_rvalue should reject non-input ranges"
struct OutputRvalueIterator {
using difference_type = int;
int operator*() const;
OutputRvalueIterator& operator++();
void operator++(int);
};
using OutputRvalueRange = decltype(ranges::subrange{OutputRvalueIterator{}, unreachable_sentinel});

static_assert(!CanViewAsRvalue<OutputRvalueRange>);
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int main() {
{ // Validate views
// ... copyable
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