[libcxx-commits] [libcxx] [libc++] Implement LWG3601: `common_iterator`'s postfix-proxy needs `indirectly_readable` (PR #237574)

[R. Aoto via libcxx-commits]

https://github.com/aotodev created https://github.com/llvm/llvm-project/pull/237574

`common_iterator::operator++(int)` now returns the postfix-proxy only when the underlying iterator is `indirectly_readable`. Previously, a non-readable iterator with a usable `value_type` and a non-referenceable `*it++` got a proxy holding a copied value. Now it gets the result of the underlying `it++`.

Fixes #104946

>From b234b61f6de5527830fea98ef1ec5c25a2a5701a Mon Sep 17 00:00:00 2001
From: Aoto <aotodev at gmail.com>
Date: Sat, 31 Oct 2026 15:58:29 +0100
Subject: [PATCH] [libc++] Implement LWG3601: `common_iterator`'s postfix-proxy
 needs `indirectly_readable`

---
 libcxx/docs/Status/Cxx23Issues.csv            |  2 +-
 libcxx/include/__iterator/common_iterator.h   |  3 ++-
 .../iterators.common/plus_plus.pass.cpp       | 17 ++++++++++++++
 .../predef.iterators/iterators.common/types.h | 22 +++++++++++++++++++
 4 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/libcxx/docs/Status/Cxx23Issues.csv b/libcxx/docs/Status/Cxx23Issues.csv
index b6f118f03e0fd9..369fbecde82fa2 100644
--- a/libcxx/docs/Status/Cxx23Issues.csv
+++ b/libcxx/docs/Status/Cxx23Issues.csv
@@ -141,7 +141,7 @@
 "`LWG3471 <https://wg21.link/LWG3471>`__","``polymorphic_allocator::allocate`` does not satisfy ``Cpp17Allocator`` requirements","2022-02 (Virtual)","","","`#104943 <https://github.com/llvm/llvm-project/issues/104943>`__",""
 "`LWG3525 <https://wg21.link/LWG3525>`__","``uses_allocator_construction_args`` fails to handle types convertible to ``pair``","2022-02 (Virtual)","","","`#104944 <https://github.com/llvm/llvm-project/issues/104944>`__",""
 "`LWG3598 <https://wg21.link/LWG3598>`__","``system_category().default_error_condition(0)`` is underspecified","2022-02 (Virtual)","","","`#104945 <https://github.com/llvm/llvm-project/issues/104945>`__",""
-"`LWG3601 <https://wg21.link/LWG3601>`__","common_iterator's postfix-proxy needs ``indirectly_readable`` ","2022-02 (Virtual)","","","`#104946 <https://github.com/llvm/llvm-project/issues/104946>`__",""
+"`LWG3601 <https://wg21.link/LWG3601>`__","common_iterator's postfix-proxy needs ``indirectly_readable`` ","2022-02 (Virtual)","|Complete|","24","`#104946 <https://github.com/llvm/llvm-project/issues/104946>`__",""
 "`LWG3607 <https://wg21.link/LWG3607>`__","``contiguous_iterator`` should not be allowed to have custom ``iter_move`` and ``iter_swap`` behavior","2022-02 (Virtual)","|Nothing To Do|","","`#104947 <https://github.com/llvm/llvm-project/issues/104947>`__",""
 "`LWG3610 <https://wg21.link/LWG3610>`__","``iota_view::size`` sometimes rejects integer-class types","2022-02 (Virtual)","|Complete|","23","`#104948 <https://github.com/llvm/llvm-project/issues/104948>`__",""
 "`LWG3612 <https://wg21.link/LWG3612>`__","Inconsistent pointer alignment in ``std::format`` ","2022-02 (Virtual)","|Complete|","14","`#104949 <https://github.com/llvm/llvm-project/issues/104949>`__",""
diff --git a/libcxx/include/__iterator/common_iterator.h b/libcxx/include/__iterator/common_iterator.h
index 02d4cac0e669aa..30fbe1b6dd2338 100644
--- a/libcxx/include/__iterator/common_iterator.h
+++ b/libcxx/include/__iterator/common_iterator.h
@@ -46,7 +46,8 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Iter>
 concept __can_use_postfix_proxy =
-    constructible_from<iter_value_t<_Iter>, iter_reference_t<_Iter>> && move_constructible<iter_value_t<_Iter>>;
+    indirectly_readable<_Iter> && constructible_from<iter_value_t<_Iter>, iter_reference_t<_Iter>> &&
+    move_constructible<iter_value_t<_Iter>>;
 
 template <input_or_output_iterator _Iter, sentinel_for<_Iter> _Sent>
   requires(!same_as<_Iter, _Sent> && copyable<_Iter>)
diff --git a/libcxx/test/std/iterators/predef.iterators/iterators.common/plus_plus.pass.cpp b/libcxx/test/std/iterators/predef.iterators/iterators.common/plus_plus.pass.cpp
index dad7ff5081a1f1..b2b340d73f19e1 100644
--- a/libcxx/test/std/iterators/predef.iterators/iterators.common/plus_plus.pass.cpp
+++ b/libcxx/test/std/iterators/predef.iterators/iterators.common/plus_plus.pass.cpp
@@ -95,6 +95,23 @@ void test() {
     assert(commonIter1 == commonSent1);
   }
 
+  // Case 2: not indirectly_readable (LWG3601)
+  {
+    int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
+    using Iter    = non_const_deref_iterator<int*>;
+    using Common  = std::common_iterator<Iter, sentinel_type<int*>>;
+    auto iter     = Common(Iter(buffer));
+    auto sent     = Common(sentinel_type<int*>{buffer + 8});
+
+    static_assert(!std::indirectly_readable<Iter>);
+    ASSERT_SAME_TYPE(decltype(iter++), void);
+
+    for (int i = 1; iter != sent; ++i) {
+      assert(*iter == i);
+      iter++;
+    }
+  }
+
   // Case 2: can-reference
   {
     int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
diff --git a/libcxx/test/std/iterators/predef.iterators/iterators.common/types.h b/libcxx/test/std/iterators/predef.iterators/iterators.common/types.h
index f5717f3eb3df37..28283d5aa7735b 100644
--- a/libcxx/test/std/iterators/predef.iterators/iterators.common/types.h
+++ b/libcxx/test/std/iterators/predef.iterators/iterators.common/types.h
@@ -121,6 +121,28 @@ class value_type_not_move_constructible_iterator
     constexpr void operator++(int) {++(*this);}
 };
 
+// Not indirectly_readable: operator* is non-const. Postfix ++ returns void, so *it++ is ill-formed.
+template <class It>
+class non_const_deref_iterator {
+  It it_;
+
+public:
+  using value_type      = std::iter_value_t<It>;
+  using difference_type = std::iter_difference_t<It>;
+
+  friend constexpr It base(const non_const_deref_iterator& i) { return i.it_; }
+
+  explicit constexpr non_const_deref_iterator(It it) : it_(it) {}
+
+  constexpr decltype(auto) operator*() { return *it_; }
+
+  constexpr non_const_deref_iterator& operator++() {
+    ++it_;
+    return *this;
+  }
+  constexpr void operator++(int) { ++it_; }
+};
+
 template <class It>
 class comparable_iterator
 {