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merged 2 commits into from
Oct 22, 2025
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<ranges>: Make ranges::to support creating unions #5794

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993270a
Make `ranges::to` support creating unions
frederick-vs-ja Oct 21, 2025
89dc2a3
Cite the Standard more precisely, avoid adding "(including union)".
StephanTLavavej Oct 22, 2025
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20 changes: 11 additions & 9 deletions stl/inc/__msvc_ranges_to.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1106,9 +1106,10 @@ namespace ranges {
_EXPORT_STD template <class _Container, input_range _Rng, class... _Types>
requires (!view<_Container>)
_NODISCARD constexpr _Container to(_Rng&& _Range, _Types&&... _Args) {
static_assert(!is_const_v<_Container>, "C must not be const. ([range.utility.conv.to])");
static_assert(!is_volatile_v<_Container>, "C must not be volatile. ([range.utility.conv.to])");
static_assert(is_class_v<_Container>, "C must be a class type. ([range.utility.conv.to])");
static_assert(!is_const_v<_Container>, "C must not be const. (N5014 [range.utility.conv.to]/1)");
static_assert(!is_volatile_v<_Container>, "C must not be volatile. (N5014 [range.utility.conv.to]/1)");
static_assert(is_class_v<_Container> || is_union_v<_Container>,
"C must be a class type. (N5014 [range.utility.conv.to]/1)");
if constexpr (_Ref_converts<_Rng, _Container>) {
if constexpr (constructible_from<_Container, _Rng, _Types...>) {
return _Container(_STD forward<_Rng>(_Range), _STD forward<_Types>(_Args)...);
Expand Down Expand Up @@ -1142,7 +1143,7 @@ namespace ranges {
} else {
static_assert(false, "ranges::to requires the result to be constructible from the source range, either "
"by using a suitable constructor, or by inserting each element of the range into "
"the default-constructed object. (N4981 [range.utility.conv.to]/2.1.5)");
"the default-constructed object. (N5014 [range.utility.conv.to]/2.1.5)");
}
} else if constexpr (input_range<range_reference_t<_Rng>>) {
const auto _Xform = [](auto&& _Elem) _STATIC_LAMBDA {
Expand All @@ -1153,15 +1154,15 @@ namespace ranges {
static_assert(false,
"ranges::to requires the elements of the source range to be either implicitly convertible to the "
"elements of the destination container, or be ranges themselves for ranges::to to be applied "
"recursively. (N4981 [range.utility.conv.to]/2.3)");
"recursively. (N5014 [range.utility.conv.to]/2.3)");
}
}

template <class _Container>
struct _To_class_fn {
_STL_INTERNAL_STATIC_ASSERT(!is_const_v<_Container>);
_STL_INTERNAL_STATIC_ASSERT(!is_volatile_v<_Container>);
_STL_INTERNAL_STATIC_ASSERT(is_class_v<_Container>);
_STL_INTERNAL_STATIC_ASSERT(is_class_v<_Container> || is_union_v<_Container>);
_STL_INTERNAL_STATIC_ASSERT(!view<_Container>);

template <input_range _Rng, class... _Types>
Expand All @@ -1176,9 +1177,10 @@ namespace ranges {
_EXPORT_STD template <class _Container, class... _Types>
requires (!view<_Container>)
_NODISCARD constexpr auto to(_Types&&... _Args) {
static_assert(!is_const_v<_Container>, "C must not be const. ([range.utility.conv.adaptors])");
static_assert(!is_volatile_v<_Container>, "C must not be volatile. ([range.utility.conv.adaptors])");
static_assert(is_class_v<_Container>, "C must be a class type. ([range.utility.conv.adaptors])");
static_assert(!is_const_v<_Container>, "C must not be const. (N5014 [range.utility.conv.adaptors]/1)");
static_assert(!is_volatile_v<_Container>, "C must not be volatile. (N5014 [range.utility.conv.adaptors]/1)");
static_assert(is_class_v<_Container> || is_union_v<_Container>,
"C must be a class type. (N5014 [range.utility.conv.adaptors]/1)");
return _Range_closure<_To_class_fn<_Container>, decay_t<_Types>...>{_STD forward<_Types>(_Args)...};
}

Expand Down
58 changes: 58 additions & 0 deletions tests/std/tests/P1206R7_ranges_to_misc/test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,7 @@
#include <memory>
#include <optional>
#include <ranges>
#include <type_traits>
#include <utility>
#include <vector>

Expand Down Expand Up @@ -160,6 +161,60 @@ constexpr bool test_nested_range() {
return true;
}

template <class T, class A = std::allocator<T>>
union union_vector {
std::vector<T, A> vec_;

constexpr union_vector() : vec_() {}
constexpr union_vector(const union_vector& other) : vec_(other.vec_) {}
constexpr union_vector(union_vector&& other) noexcept : vec_(std::move(other.vec_)) {}

template <class U>
requires (!std::same_as<std::remove_cvref_t<U>, union_vector>)
&& (!std::same_as<std::remove_cvref_t<U>, std::vector<T, A>>)
&& requires(U&& u) { std::vector<T, A>(std::forward<U>(u)); }
constexpr explicit union_vector(U&& u) : vec_(std::forward<U>(u)) {}

template <class T1, class T2, class... Ts>
requires requires(T1&& t1, T2&& t2, Ts&&... ts) {
std::vector<T, A>(std::forward<T1>(t1), std::forward<T2>(t2), std::forward<Ts>(ts)...);
}
constexpr union_vector(T1&& t1, T2&& t2, Ts&&... ts)
: vec_(std::forward<T1>(t1), std::forward<T2>(t2), std::forward<Ts>(ts)...) {}

constexpr union_vector& operator=(const union_vector& other) {
vec_ = other.vec_;
return *this;
}
constexpr union_vector& operator=(union_vector&& other)
noexcept(std::is_nothrow_move_assignable_v<std::vector<T, A>>) {
vec_ = std::move(other.vec_);
return *this;
}

constexpr ~union_vector() noexcept {
vec_.~vector();
}
};

template <ranges::input_range R, class A = std::allocator<ranges::range_value_t<R>>>
union_vector(std::from_range_t, R&&, A = A()) -> union_vector<ranges::range_value_t<R>, A>;

constexpr bool test_to_union() {
constexpr int src[]{42, 1729};

assert(ranges::equal(ranges::to<union_vector<long>>(src).vec_, src));
assert(ranges::equal((src | ranges::to<union_vector<long>>()).vec_, src));

static_assert(std::same_as<decltype(ranges::to<union_vector>(src)), union_vector<int>>);
assert(ranges::equal(ranges::to<union_vector>(src).vec_, src));

static_assert(std::same_as<decltype(src | ranges::to<union_vector>()), union_vector<int>>);
assert(ranges::equal((src | ranges::to<union_vector>()).vec_, src));

return true;
}

struct ContainerLike {
template <std::input_iterator Iter>
constexpr ContainerLike(Iter first, Iter last) : dist(static_cast<std::ptrdiff_t>(ranges::distance(first, last))) {}
Expand Down Expand Up @@ -370,6 +425,9 @@ int main() {
test_nested_range();
static_assert(test_nested_range());

test_to_union();
static_assert(test_to_union());

test_lwg3733();
static_assert(test_lwg3733());

Expand Down