[libc++][ranges] P3059R2: Making user-defined constructors of view iterators/sentinels private

libcxx/docs/ReleaseNotes/24.rst

@@ -39,6 +39,7 @@ Implemented Papers
 ------------------
 
 - P0493R5: Atomic minimum/maximum (`Github <https://llvm.org/PR105418>`__)
+- P3059R2: Making user-defined constructors of view iterators/sentinels private (`Github <https://github.com/llvm/llvm-project/issues/189599>`__)
 - P0792R14: ``function_ref`` : a type-erased callable reference (`Github <https://llvm.org/PR105376>`__)
 - P3948R1: ``constant_wrapper`` is the only tool needed for passing constant expressions via function arguments (`Github <https://llvm.org/PR189604>`__)
 - P3961R1: Less double indirection in ``function_ref`` (RU-220) (`Github <https://llvm.org/PR189606>`__)

libcxx/docs/Status/Cxx26Papers.csv

@@ -188,7 +188,7 @@
 "`P4140R0 <https://wg21.link/P4140R0>`__","Proposed resolution for US70-126: allow incomplete types in type_order","2026-03 (Croydon)","","","`#189595 <https://github.com/llvm/llvm-project/issues/189595>`__",""
 "`P3373R4 <https://wg21.link/P3373R4>`__","Of Operation States and Their Lifetimes","2026-03 (Croydon)","","","`#189597 <https://github.com/llvm/llvm-project/issues/189597>`__",""
 "`P3986R1 <https://wg21.link/P3986R1>`__","A Wording Strategy for Inlinable Receivers","2026-03 (Croydon)","","","`#189598 <https://github.com/llvm/llvm-project/issues/189598>`__",""
-"`P3059R2 <https://wg21.link/P3059R2>`__","Making user-defined constructors of view iterators/sentinels private","2026-03 (Croydon)","","","`#189599 <https://github.com/llvm/llvm-project/issues/189599>`__",""
+"`P3059R2 <https://wg21.link/P3059R2>`__","Making user-defined constructors of view iterators/sentinels private","2026-03 (Croydon)","|Complete|","24","`#189599 <https://github.com/llvm/llvm-project/issues/189599>`__",""
 "`P3725R3 <https://wg21.link/P3725R3>`__","Filter View Extensions for Safer Use, Rev 3","2026-03 (Croydon)","","","`#189601 <https://github.com/llvm/llvm-project/issues/189601>`__",""
 "`P3828R1 <https://wg21.link/P3828R1>`__","Rename the to_input view to as_input","2026-03 (Croydon)","","","`#189602 <https://github.com/llvm/llvm-project/issues/189602>`__",""
 "`P3795R2 <https://wg21.link/P3795R2>`__","Miscellaneous Reflection Cleanup","2026-03 (Croydon)","","","`#189603 <https://github.com/llvm/llvm-project/issues/189603>`__",""

libcxx/include/__ranges/elements_view.h

@@ -165,6 +165,8 @@ template <input_range _View, size_t _Np>
 template <bool _Const>
 class elements_view<_View, _Np>::__iterator
     : public __elements_view_iterator_category_base<__maybe_const<_Const, _View>, _Np> {
+  friend class elements_view<_View, _Np>;
+
   template <bool>
   friend class __iterator;
 
@@ -175,6 +177,8 @@ class elements_view<_View, _Np>::__iterator
 
   iterator_t<_Base> __current_ = iterator_t<_Base>();
 
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __iterator(iterator_t<_Base> __current) : __current_(std::move(__current)) {}
+
   _LIBCPP_HIDE_FROM_ABI static constexpr decltype(auto) __get_element(const iterator_t<_Base>& __i) {
     if constexpr (is_reference_v<range_reference_t<_Base>>) {
       return std::get<_Np>(*__i);
@@ -205,8 +209,6 @@ class elements_view<_View, _Np>::__iterator
     requires default_initializable<iterator_t<_Base>>
   = default;
 
-  _LIBCPP_HIDE_FROM_ABI constexpr explicit __iterator(iterator_t<_Base> __current) : __current_(std::move(__current)) {}
-
   _LIBCPP_HIDE_FROM_ABI constexpr __iterator(__iterator<!_Const> __i)
     requires _Const && convertible_to<iterator_t<_View>, iterator_t<_Base>>
       : __current_(std::move(__i.__current_)) {}
@@ -339,6 +341,10 @@ class elements_view<_View, _Np>::__sentinel {
   using _Base _LIBCPP_NODEBUG                        = __maybe_const<_Const, _View>;
   _LIBCPP_NO_UNIQUE_ADDRESS sentinel_t<_Base> __end_ = sentinel_t<_Base>();
 
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(sentinel_t<_Base> __end) : __end_(std::move(__end)) {}
+
+  friend class elements_view<_View, _Np>;
+
   template <bool>
   friend class __sentinel;
 
@@ -350,8 +356,6 @@ class elements_view<_View, _Np>::__sentinel {
 public:
   _LIBCPP_HIDE_FROM_ABI __sentinel() = default;
 
-  _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(sentinel_t<_Base> __end) : __end_(std::move(__end)) {}
-
   _LIBCPP_HIDE_FROM_ABI constexpr __sentinel(__sentinel<!_Const> __other)
     requires _Const && convertible_to<sentinel_t<_View>, sentinel_t<_Base>>
       : __end_(std::move(__other.__end_)) {}

libcxx/include/__ranges/filter_view.h

@@ -127,10 +127,16 @@ struct __filter_iterator_category<_View> {
 template <input_range _View, indirect_unary_predicate<iterator_t<_View>> _Pred>
   requires view<_View> && is_object_v<_Pred>
 class filter_view<_View, _Pred>::__iterator : public __filter_iterator_category<_View> {
-public:
+private:
   _LIBCPP_NO_UNIQUE_ADDRESS iterator_t<_View> __current_ = iterator_t<_View>();
   _LIBCPP_NO_UNIQUE_ADDRESS filter_view* __parent_       = nullptr;
 
+  _LIBCPP_HIDE_FROM_ABI constexpr __iterator(filter_view& __parent, iterator_t<_View> __current)
+      : __current_(std::move(__current)), __parent_(std::addressof(__parent)) {}
+
+  friend class filter_view<_View, _Pred>;
+
+public:
   using iterator_concept =
       _If<bidirectional_range<_View>,
           bidirectional_iterator_tag,
@@ -145,9 +151,6 @@ class filter_view<_View, _Pred>::__iterator : public __filter_iterator_category<
     requires default_initializable<iterator_t<_View>>
   = default;
 
-  _LIBCPP_HIDE_FROM_ABI constexpr __iterator(filter_view& __parent, iterator_t<_View> __current)
-      : __current_(std::move(__current)), __parent_(std::addressof(__parent)) {}
-
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr iterator_t<_View> const& base() const& noexcept { return __current_; }
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr iterator_t<_View> base() && { return std::move(__current_); }
 
@@ -211,13 +214,16 @@ class filter_view<_View, _Pred>::__iterator : public __filter_iterator_category<
 template <input_range _View, indirect_unary_predicate<iterator_t<_View>> _Pred>
   requires view<_View> && is_object_v<_Pred>
 class filter_view<_View, _Pred>::__sentinel {
-public:
+private:
   sentinel_t<_View> __end_ = sentinel_t<_View>();
 
-  _LIBCPP_HIDE_FROM_ABI __sentinel() = default;
-
   _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(filter_view& __parent) : __end_(ranges::end(__parent.__base_)) {}
 
+  friend class filter_view<_View, _Pred>;
+
+public:
+  _LIBCPP_HIDE_FROM_ABI __sentinel() = default;
+
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr sentinel_t<_View> base() const { return __end_; }
 
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(__iterator const& __x, __sentinel const& __y) {

libcxx/include/__ranges/iota_view.h

@@ -110,6 +110,12 @@ template <weakly_incrementable _Start, semiregular _BoundSentinel = unreachable_
   requires __weakly_equality_comparable_with<_Start, _BoundSentinel> && copyable<_Start>
 class iota_view : public view_interface<iota_view<_Start, _BoundSentinel>> {
   struct __iterator : public __iota_iterator_category<_Start> {
+  private:
+    _Start __value_ = _Start();
+
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __iterator(_Start __value) : __value_(std::move(__value)) {}
+
+  public:
     friend class iota_view;
 
     using iterator_concept =
@@ -124,14 +130,10 @@ class iota_view : public view_interface<iota_view<_Start, _BoundSentinel>> {
     using value_type      = _Start;
     using difference_type = _IotaDiffT<_Start>;
 
-    _Start __value_ = _Start();
-
     _LIBCPP_HIDE_FROM_ABI __iterator()
       requires default_initializable<_Start>
     = default;
 
-    _LIBCPP_HIDE_FROM_ABI constexpr explicit __iterator(_Start __value) : __value_(std::move(__value)) {}
-
     [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Start operator*() const
         noexcept(is_nothrow_copy_constructible_v<_Start>) {
       return __value_;
@@ -282,11 +284,12 @@ class iota_view : public view_interface<iota_view<_Start, _BoundSentinel>> {
   private:
     _BoundSentinel __bound_sentinel_ = _BoundSentinel();
 
-  public:
-    _LIBCPP_HIDE_FROM_ABI __sentinel() = default;
     _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(_BoundSentinel __bound_sentinel)
         : __bound_sentinel_(std::move(__bound_sentinel)) {}
 
+  public:
+    _LIBCPP_HIDE_FROM_ABI __sentinel() = default;
+
     _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const __iterator& __x, const __sentinel& __y) {
       return __x.__value_ == __y.__bound_sentinel_;
     }

libcxx/include/__ranges/istream_view.h

@@ -66,9 +66,6 @@ class basic_istream_view<_Val, _CharT, _Traits>::__iterator {
   using difference_type  = ptrdiff_t;
   using value_type       = _Val;
 
-  _LIBCPP_HIDE_FROM_ABI constexpr explicit __iterator(basic_istream_view<_Val, _CharT, _Traits>& __parent) noexcept
-      : __parent_(std::addressof(__parent)) {}
-
   __iterator(const __iterator&)                  = delete;
   _LIBCPP_HIDE_FROM_ABI __iterator(__iterator&&) = default;
 
@@ -91,9 +88,14 @@ class basic_istream_view<_Val, _CharT, _Traits>::__iterator {
 private:
   basic_istream_view<_Val, _CharT, _Traits>* __parent_;
 
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __iterator(basic_istream_view<_Val, _CharT, _Traits>& __parent) noexcept
+      : __parent_(std::addressof(__parent)) {}
+
   _LIBCPP_HIDE_FROM_ABI constexpr basic_istream<_CharT, _Traits>* __get_parent_stream() const {
     return __parent_->__stream_;
   }
+
+  friend class basic_istream_view<_Val, _CharT, _Traits>;
 };
 
 template <class _Val>

libcxx/include/__ranges/join_view.h

@@ -160,11 +160,13 @@ struct join_view<_View>::__sentinel {
   using _Base _LIBCPP_NODEBUG   = __maybe_const<_Const, _View>;
   sentinel_t<_Base> __end_      = sentinel_t<_Base>();
 
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(_Parent& __parent) : __end_(ranges::end(__parent.__base_)) {}
+
+  friend class join_view<_View>;
+
 public:
   _LIBCPP_HIDE_FROM_ABI __sentinel() = default;
 
-  _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(_Parent& __parent) : __end_(ranges::end(__parent.__base_)) {}
-
   _LIBCPP_HIDE_FROM_ABI constexpr __sentinel(__sentinel<!_Const> __s)
     requires _Const && convertible_to<sentinel_t<_View>, sentinel_t<_Base>>
       : __end_(std::move(__s.__end_)) {}

libcxx/include/__ranges/lazy_split_view.h

@@ -143,6 +143,8 @@ class lazy_split_view : public view_interface<lazy_split_view<_View, _Pattern>>
   template <bool _Const>
   struct __outer_iterator : __outer_iterator_category<__maybe_const<_Const, _View>> {
   private:
+    friend class lazy_split_view<_View, _Pattern>;
+
     template <bool>
     friend struct __inner_iterator;
     friend __outer_iterator<true>;
@@ -155,6 +157,14 @@ class lazy_split_view : public view_interface<lazy_split_view<_View, _Pattern>>
     _LIBCPP_NO_UNIQUE_ADDRESS _MaybeCurrent __current_ = _MaybeCurrent();
     bool __trailing_empty_                             = false;
 
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __outer_iterator(_Parent& __parent)
+      requires(!forward_range<_Base>)
+        : __parent_(std::addressof(__parent)) {}
+
+    _LIBCPP_HIDE_FROM_ABI constexpr __outer_iterator(_Parent& __parent, iterator_t<_Base> __current)
+      requires forward_range<_Base>
+        : __parent_(std::addressof(__parent)), __current_(std::move(__current)) {}
+
     [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto& __current() noexcept {
       if constexpr (forward_range<_View>) {
         return __current_;
@@ -196,14 +206,6 @@ class lazy_split_view : public view_interface<lazy_split_view<_View, _Pattern>>
 
     _LIBCPP_HIDE_FROM_ABI __outer_iterator() = default;
 
-    _LIBCPP_HIDE_FROM_ABI constexpr explicit __outer_iterator(_Parent& __parent)
-      requires(!forward_range<_Base>)
-        : __parent_(std::addressof(__parent)) {}
-
-    _LIBCPP_HIDE_FROM_ABI constexpr __outer_iterator(_Parent& __parent, iterator_t<_Base> __current)
-      requires forward_range<_Base>
-        : __parent_(std::addressof(__parent)), __current_(std::move(__current)) {}
-
     _LIBCPP_HIDE_FROM_ABI constexpr __outer_iterator(__outer_iterator<!_Const> __i)
       requires _Const && convertible_to<iterator_t<_View>, iterator_t<_Base>>
         : __parent_(__i.__parent_), __current_(std::move(__i.__current_)) {}
@@ -286,12 +288,16 @@ class lazy_split_view : public view_interface<lazy_split_view<_View, _Pattern>>
   template <bool _Const>
   struct __inner_iterator : __inner_iterator_category<__maybe_const<_Const, _View>> {
   private:
+    friend class lazy_split_view<_View, _Pattern>;
+
     using _Base _LIBCPP_NODEBUG = __maybe_const<_Const, _View>;
     // Workaround for a GCC issue.
     static constexpr bool _OuterConst = _Const;
     __outer_iterator<_Const> __i_     = __outer_iterator<_OuterConst>();
     bool __incremented_               = false;
 
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __inner_iterator(__outer_iterator<_Const> __i) : __i_(std::move(__i)) {}
+
     // Note: these private functions are necessary because GCC doesn't allow calls to private members of `__i_` from
     // free functions that are friends of `inner-iterator`.
 
@@ -342,8 +348,6 @@ class lazy_split_view : public view_interface<lazy_split_view<_View, _Pattern>>
 
     _LIBCPP_HIDE_FROM_ABI __inner_iterator() = default;
 
-    _LIBCPP_HIDE_FROM_ABI constexpr explicit __inner_iterator(__outer_iterator<_Const> __i) : __i_(std::move(__i)) {}
-
     [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const iterator_t<_Base>& base() const& noexcept {
       return __i_.__current();
     }

libcxx/include/__ranges/split_view.h

@@ -127,6 +127,12 @@ struct split_view<_View, _Pattern>::__iterator {
   _LIBCPP_NO_UNIQUE_ADDRESS subrange<iterator_t<_View>> __next_ = subrange<iterator_t<_View>>();
   bool __trailing_empty_                                        = false;
 
+  _LIBCPP_HIDE_FROM_ABI constexpr __iterator(
+      split_view<_View, _Pattern>& __parent, iterator_t<_View> __current, subrange<iterator_t<_View>> __next)
+      : __parent_(std::addressof(__parent)), __cur_(std::move(__current)), __next_(std::move(__next)) {}
+
+  friend class split_view<_View, _Pattern>;
+
   friend struct __sentinel;
 
 public:
@@ -137,10 +143,6 @@ struct split_view<_View, _Pattern>::__iterator {
 
   _LIBCPP_HIDE_FROM_ABI __iterator() = default;
 
-  _LIBCPP_HIDE_FROM_ABI constexpr __iterator(
-      split_view<_View, _Pattern>& __parent, iterator_t<_View> __current, subrange<iterator_t<_View>> __next)
-      : __parent_(std::addressof(__parent)), __cur_(std::move(__current)), __next_(std::move(__next)) {}
-
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr iterator_t<_View> base() const { return __cur_; }
 
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr value_type operator*() const { return {__cur_, __next_.begin()}; }
@@ -179,16 +181,18 @@ struct split_view<_View, _Pattern>::__sentinel {
 private:
   _LIBCPP_NO_UNIQUE_ADDRESS sentinel_t<_View> __end_ = sentinel_t<_View>();
 
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(split_view<_View, _Pattern>& __parent)
+      : __end_(ranges::end(__parent.__base_)) {}
+
   _LIBCPP_HIDE_FROM_ABI static constexpr bool __equals(const __iterator& __x, const __sentinel& __y) {
     return __x.__cur_ == __y.__end_ && !__x.__trailing_empty_;
   }
 
+  friend class split_view<_View, _Pattern>;
+
 public:
   _LIBCPP_HIDE_FROM_ABI __sentinel() = default;
 
-  _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(split_view<_View, _Pattern>& __parent)
-      : __end_(ranges::end(__parent.__base_)) {}
-
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const __iterator& __x, const __sentinel& __y) {
     return __equals(__x, __y);
   }

libcxx/include/__ranges/take_view.h

@@ -165,14 +165,16 @@ class take_view<_View>::__sentinel {
   using _Iter _LIBCPP_NODEBUG                        = counted_iterator<iterator_t<__maybe_const<_OtherConst, _View>>>;
   _LIBCPP_NO_UNIQUE_ADDRESS sentinel_t<_Base> __end_ = sentinel_t<_Base>();
 
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(sentinel_t<_Base> __end) : __end_(std::move(__end)) {}
+
+  friend class take_view<_View>;
+
   template <bool>
   friend class take_view<_View>::__sentinel;
 
 public:
   _LIBCPP_HIDE_FROM_ABI __sentinel() = default;
 
-  _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(sentinel_t<_Base> __end) : __end_(std::move(__end)) {}
-
   _LIBCPP_HIDE_FROM_ABI constexpr __sentinel(__sentinel<!_Const> __s)
     requires _Const && convertible_to<sentinel_t<_View>, sentinel_t<_Base>>
       : __end_(std::move(__s.__end_)) {}

libcxx/include/__ranges/take_while_view.h

@@ -108,14 +108,16 @@ class take_while_view<_View, _Pred>::__sentinel {
   sentinel_t<_Base> __end_ = sentinel_t<_Base>();
   const _Pred* __pred_     = nullptr;
 
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(sentinel_t<_Base> __end, const _Pred* __pred)
+      : __end_(std::move(__end)), __pred_(__pred) {}
+
+  friend class take_while_view<_View, _Pred>;
+
   friend class __sentinel<!_Const>;
 
 public:
   _LIBCPP_HIDE_FROM_ABI __sentinel() = default;
 
-  _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(sentinel_t<_Base> __end, const _Pred* __pred)
-      : __end_(std::move(__end)), __pred_(__pred) {}
-
   _LIBCPP_HIDE_FROM_ABI constexpr __sentinel(__sentinel<!_Const> __s)
     requires _Const && convertible_to<sentinel_t<_View>, sentinel_t<_Base>>
       : __end_(std::move(__s.__end_)), __pred_(__s.__pred_) {}

libcxx/include/__ranges/transform_view.h

@@ -192,6 +192,11 @@ class transform_view<_View, _Fn>::__iterator
 
   _Parent* __parent_ = nullptr;
 
+  _LIBCPP_HIDE_FROM_ABI constexpr __iterator(_Parent& __parent, iterator_t<_Base> __current)
+      : __parent_(std::addressof(__parent)), __current_(std::move(__current)) {}
+
+  friend class transform_view<_View, _Fn>;
+
   template <bool>
   friend class transform_view<_View, _Fn>::__iterator;
 
@@ -209,9 +214,6 @@ class transform_view<_View, _Fn>::__iterator
     requires default_initializable<iterator_t<_Base>>
   = default;
 
-  _LIBCPP_HIDE_FROM_ABI constexpr __iterator(_Parent& __parent, iterator_t<_Base> __current)
-      : __parent_(std::addressof(__parent)), __current_(std::move(__current)) {}
-
   // Note: `__i` should always be `__iterator<false>`, but directly using
   // `__iterator<false>` is ill-formed when `_Const` is false
   // (see http://wg21.link/class.copy.ctor#5).
@@ -354,6 +356,10 @@ class transform_view<_View, _Fn>::__sentinel {
 
   sentinel_t<_Base> __end_ = sentinel_t<_Base>();
 
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(sentinel_t<_Base> __end) : __end_(__end) {}
+
+  friend class transform_view<_View, _Fn>;
+
   template <bool>
   friend class transform_view<_View, _Fn>::__iterator;
 
@@ -363,8 +369,6 @@ class transform_view<_View, _Fn>::__sentinel {
 public:
   _LIBCPP_HIDE_FROM_ABI __sentinel() = default;
 
-  _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(sentinel_t<_Base> __end) : __end_(__end) {}
-
   // Note: `__i` should always be `__sentinel<false>`, but directly using
   // `__sentinel<false>` is ill-formed when `_Const` is false
   // (see http://wg21.link/class.copy.ctor#5).