[libcxx-commits] [libcxx] [libc++] LWG4125: `move_iterator`'s default constructor should be constrained (PR #206427)
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Mon Jun 29 01:28:10 PDT 2026
llvmorg-github-actions[bot] wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libcxx
Author: A. Jiang (frederick-vs-ja)
<details>
<summary>Changes</summary>
- The default constructor is not yet defaulted as indicated by the resolution of LWG4125, because the effects of defaulted-ness is not strictly observable. Also, it's arguable not feasible to use default member initializer `= _Iter()` because it might cause non-conforming behavior in pre-C++17 modes, due to lack of guaranteed RVO.
- In the test file, `NoDefaultCtr` is replaced with a proper iterator type `cpp20_input_iterator<int*>` to make the test more pedantically conforming.
Fixes #<!-- -->204442.
---
Full diff: https://github.com/llvm/llvm-project/pull/206427.diff
3 Files Affected:
- (modified) libcxx/docs/Status/Cxx29Issues.csv (+1-1)
- (modified) libcxx/include/__iterator/move_iterator.h (+2-2)
- (modified) libcxx/test/std/iterators/predef.iterators/move.iterators/move.iter.ops/move.iter.op.const/default.pass.cpp (+27-10)
``````````diff
diff --git a/libcxx/docs/Status/Cxx29Issues.csv b/libcxx/docs/Status/Cxx29Issues.csv
index d9e245a9a11c0..13056a97782df 100644
--- a/libcxx/docs/Status/Cxx29Issues.csv
+++ b/libcxx/docs/Status/Cxx29Issues.csv
@@ -1,7 +1,7 @@
"Issue #","Issue Name","Meeting","Status","First released version","GitHub issue","Notes"
"`LWG3417 <https://wg21.link/LWG3417>`__","Missing ``volatile`` atomic deprecations","2026-06 (Brno)","","","`#204440 <https://github.com/llvm/llvm-project/issues/204440>`__",""
"`LWG4121 <https://wg21.link/LWG4121>`__","``ranges::to`` constructs associative containers via ``c.emplace(c.end(), *it)``","2026-06 (Brno)","","","`#204441 <https://github.com/llvm/llvm-project/issues/204441>`__",""
-"`LWG4125 <https://wg21.link/LWG4125>`__","``move_iterator``'s default constructor should be constrained","2026-06 (Brno)","","","`#204442 <https://github.com/llvm/llvm-project/issues/204442>`__",""
+"`LWG4125 <https://wg21.link/LWG4125>`__","``move_iterator``'s default constructor should be constrained","2026-06 (Brno)","|Complete|","23","`#204442 <https://github.com/llvm/llvm-project/issues/204442>`__",""
"`LWG4158 <https://wg21.link/LWG4158>`__","``packaged_task::operator=`` should abandon its shared state","2026-06 (Brno)","","","`#204443 <https://github.com/llvm/llvm-project/issues/204443>`__",""
"`LWG4182 <https://wg21.link/LWG4182>`__","Definition of ``NULL`` is too broad","2026-06 (Brno)","","","`#204444 <https://github.com/llvm/llvm-project/issues/204444>`__",""
"`LWG4218 <https://wg21.link/LWG4218>`__","Constraint recursion in ``basic_const_iterator``'s relational operators due to ADL + CWG 2369","2026-06 (Brno)","","","`#204445 <https://github.com/llvm/llvm-project/issues/204445>`__",""
diff --git a/libcxx/include/__iterator/move_iterator.h b/libcxx/include/__iterator/move_iterator.h
index c42f4cb53e9a8..aa5570c018737 100644
--- a/libcxx/include/__iterator/move_iterator.h
+++ b/libcxx/include/__iterator/move_iterator.h
@@ -13,6 +13,7 @@
#include <__compare/compare_three_way_result.h>
#include <__compare/three_way_comparable.h>
#include <__concepts/assignable.h>
+#include <__concepts/constructible.h>
#include <__concepts/convertible_to.h>
#include <__concepts/derived_from.h>
#include <__concepts/same_as.h>
@@ -27,7 +28,6 @@
#include <__type_traits/conditional.h>
#include <__type_traits/enable_if.h>
#include <__type_traits/is_assignable.h>
-#include <__type_traits/is_constructible.h>
#include <__type_traits/is_convertible.h>
#include <__type_traits/is_reference.h>
#include <__type_traits/is_same.h>
@@ -122,7 +122,7 @@ class move_iterator
#if _LIBCPP_STD_VER >= 20
_LIBCPP_HIDE_FROM_ABI constexpr move_iterator()
- requires is_constructible_v<_Iter>
+ requires default_initializable<_Iter>
: __current_() {}
template <class _Up>
diff --git a/libcxx/test/std/iterators/predef.iterators/move.iterators/move.iter.ops/move.iter.op.const/default.pass.cpp b/libcxx/test/std/iterators/predef.iterators/move.iterators/move.iter.ops/move.iter.op.const/default.pass.cpp
index d095182c560d8..00512fa2f3dad 100644
--- a/libcxx/test/std/iterators/predef.iterators/move.iterators/move.iter.ops/move.iter.op.const/default.pass.cpp
+++ b/libcxx/test/std/iterators/predef.iterators/move.iterators/move.iter.ops/move.iter.op.const/default.pass.cpp
@@ -10,11 +10,8 @@
// move_iterator
-// move_iterator();
-//
-// constexpr in C++17
-//
-// requires the underlying iterator to be default-constructible (extension).
+// constexpr move_iterator(); // constexpr since C++17
+// requires default_initializable<Iterator>; // since C++20
#include <iterator>
@@ -22,13 +19,33 @@
#include "test_macros.h"
#include "test_iterators.h"
-#if TEST_STD_VER > 17
-struct NoDefaultCtr : forward_iterator<int*> {
- NoDefaultCtr() = delete;
+#if TEST_STD_VER >= 20
+class constable_iter {
+public:
+ using iterator_category = std::input_iterator_tag;
+ using difference_type = std::ptrdiff_t;
+ using value_type = char;
+
+ constable_iter() = default;
+
+ template <class U = constable_iter>
+ const constable_iter& operator=(const std::type_identity_t<constable_iter>&) const;
+
+ char operator*() const;
+
+ const constable_iter& operator++() const;
+ void operator++(int) const;
+
+private:
+ char payload_; // making `const constable_iter` value-initializable but not default-initializable
};
+static_assert(std::is_default_constructible_v<const constable_iter>);
+static_assert(!std::default_initializable<const constable_iter>);
+static_assert(std::input_iterator<const constable_iter>);
-LIBCPP_STATIC_ASSERT( std::is_default_constructible_v<std::move_iterator<forward_iterator<int*>>>);
-LIBCPP_STATIC_ASSERT(!std::is_default_constructible_v<std::move_iterator<NoDefaultCtr>>);
+static_assert(std::is_default_constructible_v<std::move_iterator<forward_iterator<int*>>>);
+static_assert(!std::is_default_constructible_v<std::move_iterator<cpp20_input_iterator<int*>>>);
+static_assert(!std::is_default_constructible_v<std::move_iterator<const constable_iter>>);
#endif
template <class It>
``````````
</details>
https://github.com/llvm/llvm-project/pull/206427
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