[libc++] Split ABI flag for removing iterator bases and removing the second member in reverse_iterator (#143079)

Currently `_LIBCPP_NO_ITERATOR_BASES` controls both whether specific classes derive from `iterator` and whether `reverse_iterator` has a second member variable. These two changes are orthogonal though, and one can be applied in all langauge modes while the other change is only conforming for C++17 and later.
2025-09-04 09:18:49 +02:00 · 2025-09-04 09:18:49 +02:00 · e8fa13ca4e
commit e8fa13ca4e
parent cd44260ee6
13 changed files with 61 additions and 69 deletions
--- a/libcxx/docs/ABIGuarantees.rst
+++ b/libcxx/docs/ABIGuarantees.rst
@ -44,6 +44,10 @@ This removes the ``iterator`` base class from ``back_insert_iterator``, ``front_
 This doesn't directly affect the layout of these types in most cases, but may result in more padding being used when
 they are used in combination, for example ``reverse_iterator<reverse_iterator<T>>``.

+``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER``
+-------------------------------------------------
+This removes a second member in ``reverse_iterator`` that is unused after LWG2360.
+
 ``_LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION``
 -------------------------------------------------
 This changes the index type used inside ``variant`` to the smallest required type to reduce the datasize of variants in
--- a/libcxx/docs/ReleaseNotes/22.rst
+++ b/libcxx/docs/ReleaseNotes/22.rst
@ -75,11 +75,19 @@ Potentially breaking changes
  first element being returned from ``find`` will be broken, and ``lower_bound`` or ``equal_range`` should be used
  instead.

+- The ABI flag ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` has been split off from
+  ``_LIBCPP_ABI_NO_ITERATOR_BASES``. If you are using this flag and care about ABI stability, you should set
+  ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` as well.
+
 Announcements About Future Releases
 -----------------------------------

 ABI Affecting Changes
 ---------------------

+- The ABI flag ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` has been split off from
+  ``_LIBCPP_ABI_NO_ITERATOR_BASES``. If you are using this flag and care about ABI stability, you should set
+  ``_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER`` as well.
+
 Build System Changes
 --------------------
--- a/libcxx/include/__configuration/abi.h
+++ b/libcxx/include/__configuration/abi.h
@ -72,6 +72,7 @@
 #  define _LIBCPP_ABI_NO_FILESYSTEM_INLINE_NAMESPACE
 #  define _LIBCPP_ABI_NO_ITERATOR_BASES
 #  define _LIBCPP_ABI_NO_RANDOM_DEVICE_COMPATIBILITY_LAYOUT
+#  define _LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER
 #  define _LIBCPP_ABI_OPTIMIZED_FUNCTION
 #  define _LIBCPP_ABI_REGEX_CONSTANTS_NONZERO
 #  define _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATION
@ -98,6 +99,15 @@
 #  endif
 #endif

+// TODO(LLVM 22): Remove this check
+#if defined(_LIBCPP_ABI_NO_ITERATOR_BASES) && !defined(_LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER)
+#  ifndef _LIBCPP_ONLY_NO_ITERATOR_BASES
+#    error "You probably want to define _LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER. This has been split out from"   \
+ " _LIBCPP_ABI_NO_ITERATOR_BASES to allow only removing the second iterator member, since they aren't really related." \
+ "If you actually want this ABI configuration, please define _LIBCPP_ONLY_NO_ITERATOR_BASES instead."
+#  endif
+#endif
+
 // We had some bugs where we use [[no_unique_address]] together with construct_at,
 // which causes UB as the call on construct_at could write to overlapping subobjects
 //
--- a/libcxx/include/__iterator/back_insert_iterator.h
+++ b/libcxx/include/__iterator/back_insert_iterator.h
@ -26,15 +26,9 @@ _LIBCPP_PUSH_MACROS

 _LIBCPP_BEGIN_NAMESPACE_STD

-_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <class _Container>
 class back_insert_iterator
-#if _LIBCPP_STD_VER <= 14 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
-    : public iterator<output_iterator_tag, void, void, void, void>
-#endif
-{
-  _LIBCPP_SUPPRESS_DEPRECATED_POP
-
+    : public __iterator_base<back_insert_iterator<_Container>, output_iterator_tag, void, void, void, void> {
 protected:
  _Container* container;

--- a/libcxx/include/__iterator/front_insert_iterator.h
+++ b/libcxx/include/__iterator/front_insert_iterator.h
@ -26,15 +26,9 @@ _LIBCPP_PUSH_MACROS

 _LIBCPP_BEGIN_NAMESPACE_STD

-_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <class _Container>
 class front_insert_iterator
-#if _LIBCPP_STD_VER <= 14 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
-    : public iterator<output_iterator_tag, void, void, void, void>
-#endif
-{
-  _LIBCPP_SUPPRESS_DEPRECATED_POP
-
+    : public __iterator_base<front_insert_iterator<_Container>, output_iterator_tag, void, void, void, void> {
 protected:
  _Container* container;

--- a/libcxx/include/__iterator/insert_iterator.h
+++ b/libcxx/include/__iterator/insert_iterator.h
@ -35,15 +35,9 @@ template <class _Container>
 using __insert_iterator_iter_t _LIBCPP_NODEBUG = typename _Container::iterator;
 #endif

-_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <class _Container>
 class insert_iterator
-#if _LIBCPP_STD_VER <= 14 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
-    : public iterator<output_iterator_tag, void, void, void, void>
-#endif
-{
-  _LIBCPP_SUPPRESS_DEPRECATED_POP
-
+    : public __iterator_base<insert_iterator<_Container>, output_iterator_tag, void, void, void, void> {
 protected:
  _Container* container;
  __insert_iterator_iter_t<_Container> iter;
--- a/libcxx/include/__iterator/istream_iterator.h
+++ b/libcxx/include/__iterator/istream_iterator.h
@ -25,15 +25,14 @@

 _LIBCPP_BEGIN_NAMESPACE_STD

-_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <class _Tp, class _CharT = char, class _Traits = char_traits<_CharT>, class _Distance = ptrdiff_t>
 class istream_iterator
-#if _LIBCPP_STD_VER <= 14 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
-    : public iterator<input_iterator_tag, _Tp, _Distance, const _Tp*, const _Tp&>
-#endif
-{
-  _LIBCPP_SUPPRESS_DEPRECATED_POP
-
+    : public __iterator_base<istream_iterator<_Tp, _CharT, _Traits, _Distance>,
+                             input_iterator_tag,
+                             _Tp,
+                             _Distance,
+                             const _Tp*,
+                             const _Tp&> {
 public:
  typedef input_iterator_tag iterator_category;
  typedef _Tp value_type;
--- a/libcxx/include/__iterator/istreambuf_iterator.h
+++ b/libcxx/include/__iterator/istreambuf_iterator.h
@ -25,15 +25,14 @@

 _LIBCPP_BEGIN_NAMESPACE_STD

-_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <class _CharT, class _Traits>
 class istreambuf_iterator
-#if _LIBCPP_STD_VER <= 14 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
-    : public iterator<input_iterator_tag, _CharT, typename _Traits::off_type, _CharT*, _CharT>
-#endif
-{
-  _LIBCPP_SUPPRESS_DEPRECATED_POP
-
+    : public __iterator_base<istreambuf_iterator<_CharT, _Traits>,
+                             input_iterator_tag,
+                             _CharT,
+                             typename _Traits::off_type,
+                             _CharT*,
+                             _CharT> {
 public:
  typedef input_iterator_tag iterator_category;
  typedef _CharT value_type;
--- a/libcxx/include/__iterator/iterator.h
+++ b/libcxx/include/__iterator/iterator.h
@ -28,6 +28,19 @@ struct _LIBCPP_DEPRECATED_IN_CXX17 iterator {
  typedef _Category iterator_category;
 };

+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
+#ifdef _LIBCPP_ABI_NO_ITERATOR_BASES
+template <class _Derived>
+struct __no_iterator_base {};
+
+template <class _Derived, class _Category, class _Tp, class _Distance, class _Pointer, class _Reference>
+using __iterator_base _LIBCPP_NODEBUG = __no_iterator_base<_Derived>;
+#else
+template <class _Derived, class _Category, class _Tp, class _Distance, class _Pointer, class _Reference>
+using __iterator_base _LIBCPP_NODEBUG = iterator<_Category, _Tp, _Distance, _Pointer, _Reference>;
+#endif
+_LIBCPP_SUPPRESS_DEPRECATED_POP
+
 _LIBCPP_END_NAMESPACE_STD

 #endif // _LIBCPP___ITERATOR_ITERATOR_H
--- a/libcxx/include/__iterator/ostream_iterator.h
+++ b/libcxx/include/__iterator/ostream_iterator.h
@ -24,15 +24,9 @@

 _LIBCPP_BEGIN_NAMESPACE_STD

-_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <class _Tp, class _CharT = char, class _Traits = char_traits<_CharT> >
 class ostream_iterator
-#if _LIBCPP_STD_VER <= 14 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
-    : public iterator<output_iterator_tag, void, void, void, void>
-#endif
-{
-  _LIBCPP_SUPPRESS_DEPRECATED_POP
-
+    : public __iterator_base<ostream_iterator<_Tp, _CharT, _Traits>, output_iterator_tag, void, void, void, void> {
 public:
  typedef output_iterator_tag iterator_category;
  typedef void value_type;
--- a/libcxx/include/__iterator/ostreambuf_iterator.h
+++ b/libcxx/include/__iterator/ostreambuf_iterator.h
@ -25,15 +25,9 @@

 _LIBCPP_BEGIN_NAMESPACE_STD

-_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <class _CharT, class _Traits>
 class ostreambuf_iterator
-#if _LIBCPP_STD_VER <= 14 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
-    : public iterator<output_iterator_tag, void, void, void, void>
-#endif
-{
-  _LIBCPP_SUPPRESS_DEPRECATED_POP
-
+    : public __iterator_base<ostreambuf_iterator<_CharT, _Traits>, output_iterator_tag, void, void, void, void> {
 public:
  typedef output_iterator_tag iterator_category;
  typedef void value_type;
--- a/libcxx/include/__iterator/reverse_iterator.h
+++ b/libcxx/include/__iterator/reverse_iterator.h
@ -46,21 +46,16 @@

 _LIBCPP_BEGIN_NAMESPACE_STD

-_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <class _Iter>
 class reverse_iterator
-#if _LIBCPP_STD_VER <= 14 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
-    : public iterator<typename iterator_traits<_Iter>::iterator_category,
-                      typename iterator_traits<_Iter>::value_type,
-                      typename iterator_traits<_Iter>::difference_type,
-                      typename iterator_traits<_Iter>::pointer,
-                      typename iterator_traits<_Iter>::reference>
-#endif
-{
-  _LIBCPP_SUPPRESS_DEPRECATED_POP
-
+    : public __iterator_base<reverse_iterator<_Iter>,
+                             typename iterator_traits<_Iter>::iterator_category,
+                             typename iterator_traits<_Iter>::value_type,
+                             typename iterator_traits<_Iter>::difference_type,
+                             typename iterator_traits<_Iter>::pointer,
+                             typename iterator_traits<_Iter>::reference> {
 private:
-#ifndef _LIBCPP_ABI_NO_ITERATOR_BASES
+#ifndef _LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER
  _Iter __t_; // no longer used as of LWG #2360, not removed due to ABI break
 #endif

@ -91,7 +86,7 @@ public:
  using reference       = typename iterator_traits<_Iter>::reference;
 #endif

-#ifndef _LIBCPP_ABI_NO_ITERATOR_BASES
+#ifndef _LIBCPP_ABI_NO_REVERSE_ITERATOR_SECOND_MEMBER
  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reverse_iterator() : __t_(), current() {}

  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 explicit reverse_iterator(_Iter __x) : __t_(__x), current(__x) {}
--- a/libcxx/include/__memory/raw_storage_iterator.h
+++ b/libcxx/include/__memory/raw_storage_iterator.h
@ -28,15 +28,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD

 #if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_RAW_STORAGE_ITERATOR)

-_LIBCPP_SUPPRESS_DEPRECATED_PUSH
 template <class _OutputIterator, class _Tp>
 class _LIBCPP_DEPRECATED_IN_CXX17 raw_storage_iterator
-#  if _LIBCPP_STD_VER <= 14 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
-    : public iterator<output_iterator_tag, void, void, void, void>
-#  endif
-{
-  _LIBCPP_SUPPRESS_DEPRECATED_POP
-
+    : public __iterator_base<raw_storage_iterator<_OutputIterator, _Tp>, output_iterator_tag, void, void, void, void> {
 private:
  _OutputIterator __x_;