Automerge: [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.

Author: philnik777

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

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
 --------------------

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
 //

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;
 

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;
 

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;

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;

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;

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

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;