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Implement LWG-3797 elements_view insufficiently constrained #6310

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Implement LWG-3797 elements_view insufficiently constrained #6310

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4 changes: 3 additions & 1 deletion stl/inc/ranges
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Original file line number Diff line number Diff line change
Expand Up @@ -4706,7 +4706,9 @@ namespace ranges {
template <class _Tuple, size_t _Index>
concept _Has_tuple_element =
#if _HAS_CXX23
_Tuple_like<_Tuple> && _Index < tuple_size_v<_Tuple>;
_Tuple_like<_Tuple> && _Index < tuple_size_v<_Tuple> && requires(_Tuple __t) {
{ _STD get<_Index>(__t) } -> convertible_to<const tuple_element_t<_Index, _Tuple>&>;
};
#else // ^^^ _HAS_CXX23 / !_HAS_CXX23 vvv
requires(_Tuple __t) {
typename tuple_size<_Tuple>::type;
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26 changes: 26 additions & 0 deletions tests/std/tests/P0896R4_views_elements/test.cpp
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Expand Up @@ -414,6 +414,32 @@ void test_gh_3014() { // COMPILE-ONLY
[[maybe_unused]] decltype(as_const(r).begin()) i = r.begin(); // Check 'iterator(iterator<!Const> i)'
}

// LWG-3797 "elements_view insufficiently constrained"
namespace lwg_3797 {
struct Constifier {
template <class T>
constexpr const T& operator()(const T& t) const noexcept {
return t;
}
};

struct Mover {
template <class T>
constexpr remove_reference_t<T>&& operator()(T&& t) const noexcept {
return static_cast<remove_reference_t<T>&&>(t);
}
};

using MoveOnlySubrange = ranges::subrange<test::iterator<test::input, int>, test::sentinel<int>>;
static_assert(!CanViewElements<vector<MoveOnlySubrange>>);
static_assert(!CanViewElements<decltype(vector<MoveOnlySubrange>{} | views::transform(Constifier{}))>);
static_assert(!CanViewElements<decltype(vector<MoveOnlySubrange>{} | views::transform(Mover{}))>);
#if _HAS_CXX23
static_assert(!CanViewElements<decltype(vector<MoveOnlySubrange>{} | views::as_const)>);
static_assert(!CanViewElements<decltype(vector<MoveOnlySubrange>{} | views::as_rvalue)>);
#endif // _HAS_CXX23
} // namespace lwg_3797

int main() {
{ // Validate copyable views
constexpr span<const P> s{some_pairs};
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