[libc++] Add _LIBCPP_ABI_NO_ITERATOR_BASES. Fix input/output iterators' difference_type for C++20.

[• Quuxplusone]

Summary

This makes ostream_iterator and ostreambuf_iterator conforming to C++20, I think. (They needed to be default-constructible. But p2325 will remove the default constructor again, as a DR against C++20, when it lands.)

Supersedes D101729.

[cjdb]

libcxx/include/iterator

620–626

This is already taken care of in D101729. It's literally the point of that patch. Same anywhere else you're doing this.

[zoecarver]

WDYT about/would it be possible to add a test that would fail if we had an ABI break, but doesn't because we don't break ABI with this change (so long as we're not using the unstable ABI)?

libcxx/include/__config

97

Are there some docs we can stick this in? Maybe mention it's enabled by default for the unstable ABI (as it should be).

[ldionne]

libcxx/include/iterator

792

There is no ABI implication here AFAICT, since the first member will always begin at offset 0 anyway. So this means this can just be #if _LIBCPP_STD_VER <= 14.

Also, notice that your _LIBCPP_STD_VER is wrong, it should be compared to 14, not 11. The std::iterator bases were removed in C++17.

915

Same.

983

Same.

1027

Same.

1104

Same.

[• Quuxplusone]

libcxx/include/iterator

792

The ABI implication is the existence of the empty base (see https://quuxplusone.github.io/blog/2021/05/07/std-iterator-as-a-base-class/ ). For example,

#include <vector>
#include <iterator>
#include <stdio.h>
struct S : std::back_insert_iterator<std::vector<int>>, std::iterator<std::output_iterator_tag, void, void, void, void> {};
int main() { printf("%d\n", (int)sizeof(S)); }

returns 16 when compiled with clang++ -std=c++17 -D_LIBCPP_ABI_VERSION=1 (or before this patch), but 8 when compiled (after this patch) with clang++ -std=c++17 -D_LIBCPP_ABI_VERSION=2.
IOW, the ABI break is confined to the new ABI; the old ABI continues working the same as before.

The iterator bases were removed in C++17 [not C++14]

Darn, you're right. I had written in my post that LWG2438 was "a very late-breaking change to C++14," but I guess it was so late that it was C++17. ;) @tcanens' HTML version of N4140 (C++14) agrees that the base classes are still there in C++14. Will change.

[ldionne]

libcxx/include/iterator

792

Yeah, indeed, I just realized that you were right. This is pretty terrible.

[ldionne]

I think this can be closed now.

libcxx/include/__config

 // Don't use a nullptr_t simulation type in C++03 instead using C++11 nullptr
 // provided under the alternate keyword __nullptr, which changes the mangling
 // of nullptr_t. This option is ABI incompatible with GCC in C++03 mode.
 #  define _LIBCPP_ABI_ALWAYS_USE_CXX11_NULLPTR
 // Define a key function for `bad_function_call` in the library, to centralize
 // its vtable and typeinfo to libc++ rather than having all other libraries
 // using that class define their own copies.
 #  define _LIBCPP_ABI_BAD_FUNCTION_CALL_KEY_FUNCTION
+// Give reverse_iterator<T> one data member of type T, not two.
+// Also, in C++14 and later, don't derive iterator types from std::iterator.
+#  define _LIBCPP_ABI_NO_ITERATOR_BASES
 // Enable optimized version of __do_get_(un)signed which avoids redundant copies.
 #  define _LIBCPP_ABI_OPTIMIZED_LOCALE_NUM_GET
 // Use the smallest possible integer type to represent the index of the variant.
 // Previously libc++ used "unsigned int" exclusively.
 #  define _LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION
 // Unstable attempt to provide a more optimized std::function
 #  define _LIBCPP_ABI_OPTIMIZED_FUNCTION
 // All the regex constants must be distinct and nonzero.

libcxx/include/__memory/raw_storage_iterator.h

 #include <__undef_macros>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_RAW_STORAGE_ITERATOR)
 
 template <class _OutputIterator, class _Tp>
 class _LIBCPP_TEMPLATE_VIS _LIBCPP_DEPRECATED_IN_CXX17 raw_storage_iterator
+#if _LIBCPP_STD_VER <= 11 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
     : public iterator<output_iterator_tag,
                       _Tp,                                         // purposefully not C++03
                       ptrdiff_t,                                   // purposefully not C++03
                       _Tp*,                                        // purposefully not C++03
                       raw_storage_iterator<_OutputIterator, _Tp>&> // purposefully not C++03
+#endif
 {
 private:
     _OutputIterator __x_;
 public:
+    typedef output_iterator_tag   iterator_category;
+    typedef _Tp                   value_type;
+    typedef ptrdiff_t             difference_type;
+    typedef _Tp*                  pointer;
+    typedef raw_storage_iterator& reference;
+
     _LIBCPP_INLINE_VISIBILITY explicit raw_storage_iterator(_OutputIterator __x) : __x_(__x) {}
     _LIBCPP_INLINE_VISIBILITY raw_storage_iterator& operator*() {return *this;}
     _LIBCPP_INLINE_VISIBILITY raw_storage_iterator& operator=(const _Tp& __element)
         {::new ((void*)_VSTD::addressof(*__x_)) _Tp(__element); return *this;}
 #if _LIBCPP_STD_VER >= 14
     _LIBCPP_INLINE_VISIBILITY raw_storage_iterator& operator=(_Tp&& __element)
         {::new ((void*)_VSTD::addressof(*__x_)) _Tp(_VSTD::move(__element)); return *this;}
 #endif

libcxx/include/iterator

 class ostream_iterator
     : public iterator<output_iterator_tag, void, void, void ,void>
 {
 public:
     typedef charT char_type;
     typedef traits traits_type;
     typedef basic_ostream<charT,traits> ostream_type;
 
+    constexpr ostream_iterator() noexcept = default; // C++20
     ostream_iterator(ostream_type& s);
     ostream_iterator(ostream_type& s, const charT* delimiter);
     ostream_iterator(const ostream_iterator& x);
     ~ostream_iterator();
     ostream_iterator& operator=(const T& value);
 
     ostream_iterator& operator*();
     ostream_iterator& operator++();
class ostreambuf_iterator
     : public iterator<output_iterator_tag, void, void, void, void>
 {
 public:
     typedef charT                         char_type;
     typedef traits                        traits_type;
     typedef basic_streambuf<charT,traits> streambuf_type;
     typedef basic_ostream<charT,traits>   ostream_type;
 
+    constexpr ostreambuf_iterator() noexcept = default; // C++20
     ostreambuf_iterator(ostream_type& s) noexcept;
     ostreambuf_iterator(streambuf_type* s) noexcept;
     ostreambuf_iterator& operator=(charT c);
     ostreambuf_iterator& operator*();
     ostreambuf_iterator& operator++();
     ostreambuf_iterator& operator++(int);
     bool failed() const noexcept;
 };

 struct __is_stashing_iterator : false_type {};
 
 template <class _Tp>
 struct __is_stashing_iterator<_Tp, typename __void_t<typename _Tp::__stashing_iterator_tag>::type>
   : true_type {};
 
 template <class _Iter>
 class _LIBCPP_TEMPLATE_VIS reverse_iterator
+#if _LIBCPP_STD_VER <= 11 || !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
 {
 private:
-    /*mutable*/ _Iter __t;  // no longer used as of LWG #2360, not removed due to ABI break
+#ifndef _LIBCPP_ABI_NO_ITERATOR_BASES
+    _Iter __t;  // no longer used as of LWG #2360, not removed due to ABI break
+#endif
 
     static_assert(!__is_stashing_iterator<_Iter>::value,
       "The specified iterator type cannot be used with reverse_iterator; "
       "Using stashing iterators with reverse_iterator causes undefined behavior");
 
 protected:
     _Iter current;
 public:
     typedef _Iter                                            iterator_type;
+    typedef typename iterator_traits<_Iter>::value_type      value_type;
     typedef typename iterator_traits<_Iter>::difference_type difference_type;
     typedef typename iterator_traits<_Iter>::reference       reference;
     typedef typename iterator_traits<_Iter>::pointer         pointer;
     typedef _If<__is_cpp17_random_access_iterator<_Iter>::value,
         random_access_iterator_tag,
         typename iterator_traits<_Iter>::iterator_category>  iterator_category;
 #if _LIBCPP_STD_VER > 17
     typedef _If<__is_cpp17_random_access_iterator<_Iter>::value,
         random_access_iterator_tag,
         bidirectional_iterator_tag>                          iterator_concept;
 #endif
 
+#ifndef _LIBCPP_ABI_NO_ITERATOR_BASES
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reverse_iterator() : __t(), current() {}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     explicit reverse_iterator(_Iter __x) : __t(__x), current(__x) {}
     template <class _Up>
         _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
         reverse_iterator(const reverse_iterator<_Up>& __u) : __t(__u.base()), current(__u.base()) {}
     template <class _Up>
         _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
         reverse_iterator& operator=(const reverse_iterator<_Up>& __u)
             { __t = current = __u.base(); return *this; }
+#else
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
+    reverse_iterator() : current() {}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
+    explicit reverse_iterator(_Iter __x) : current(__x) {}
+    template <class _Up>
+        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
+        reverse_iterator(const reverse_iterator<_Up>& __u) : current(__u.base()) {}
+    template <class _Up>
+        _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
+        reverse_iterator& operator=(const reverse_iterator<_Up>& __u)
+            { current = __u.base(); return *this; }
+#endif
+
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     _Iter base() const {return current;}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reference operator*() const {_Iter __tmp = current; return *--__tmp;}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     pointer  operator->() const {return _VSTD::addressof(operator*());}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reverse_iterator& operator++() {--current; return *this;}

 reverse_iterator<_Iter> make_reverse_iterator(_Iter __i)
 {
     return reverse_iterator<_Iter>(__i);
 }
 #endif
 
 template <class _Container>
 class _LIBCPP_TEMPLATE_VIS back_insert_iterator
-    : public iterator<output_iterator_tag,
-                      void,
-                      void,
-                      void,
-                      void>
+#if _LIBCPP_STD_VER <= 11 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
+    : public iterator<output_iterator_tag, void, void, void, void>
+#endif
 {
 protected:
     _Container* container;
 public:
     typedef _Container container_type;
+    typedef output_iterator_tag iterator_category;
+    typedef void value_type;
+#if _LIBCPP_STD_VER > 17
+    typedef ptrdiff_t                       difference_type;
+#else
+    typedef void                            difference_type;
+#endif
+    typedef void pointer;
+    typedef void reference;
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 explicit back_insert_iterator(_Container& __x) : container(_VSTD::addressof(__x)) {}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator=(const typename _Container::value_type& __value_)
         {container->push_back(__value_); return *this;}
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator=(typename _Container::value_type&& __value_)
         {container->push_back(_VSTD::move(__value_)); return *this;}
 #endif // _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator*()     {return *this;}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator++()    {return *this;}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator  operator++(int) {return *this;}
 };
 
 template <class _Container>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 back_insert_iterator<_Container>
 back_inserter(_Container& __x)
 {
     return back_insert_iterator<_Container>(__x);
 }
 
 template <class _Container>
 class _LIBCPP_TEMPLATE_VIS front_insert_iterator
-    : public iterator<output_iterator_tag,
-                      void,
-                      void,
-                      void,
-                      void>
+#if _LIBCPP_STD_VER <= 11 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
+    : public iterator<output_iterator_tag, void, void, void, void>
+#endif
 {
 protected:
     _Container* container;
 public:
     typedef _Container container_type;
+    typedef output_iterator_tag iterator_category;
+    typedef void value_type;
+#if _LIBCPP_STD_VER > 17
+    typedef ptrdiff_t                       difference_type;
+#else
+    typedef void                            difference_type;
+#endif
+    typedef void pointer;
+    typedef void reference;
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 explicit front_insert_iterator(_Container& __x) : container(_VSTD::addressof(__x)) {}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 front_insert_iterator& operator=(const typename _Container::value_type& __value_)
         {container->push_front(__value_); return *this;}
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 front_insert_iterator& operator=(typename _Container::value_type&& __value_)
         {container->push_front(_VSTD::move(__value_)); return *this;}
 #endif // _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 front_insert_iterator& operator*()     {return *this;}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 front_insert_iterator& operator++()    {return *this;}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 front_insert_iterator  operator++(int) {return *this;}
 };
 
 template <class _Container>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 front_insert_iterator<_Container>
 front_inserter(_Container& __x)
 {
     return front_insert_iterator<_Container>(__x);
 }
 
 template <class _Container>
 class _LIBCPP_TEMPLATE_VIS insert_iterator
-    : public iterator<output_iterator_tag,
-                      void,
-                      void,
-                      void,
-                      void>
+#if _LIBCPP_STD_VER <= 11 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
+    : public iterator<output_iterator_tag, void, void, void, void>
+#endif
 {
 protected:
     _Container* container;
     typename _Container::iterator iter;
 public:
     typedef _Container container_type;
+    typedef output_iterator_tag iterator_category;
+    typedef void value_type;
+#if _LIBCPP_STD_VER > 17
+    typedef ptrdiff_t                       difference_type;
+#else
+    typedef void                            difference_type;
+#endif
+    typedef void pointer;
+    typedef void reference;
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 insert_iterator(_Container& __x, typename _Container::iterator __i)
         : container(_VSTD::addressof(__x)), iter(__i) {}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 insert_iterator& operator=(const typename _Container::value_type& __value_)
         {iter = container->insert(iter, __value_); ++iter; return *this;}
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 insert_iterator& operator=(typename _Container::value_type&& __value_)
         {iter = container->insert(iter, _VSTD::move(__value_)); ++iter; return *this;}

 inserter(_Container& __x, typename _Container::iterator __i)
 {
     return insert_iterator<_Container>(__x, __i);
 }
 
 template <class _Tp, class _CharT = char,
           class _Traits = char_traits<_CharT>, class _Distance = ptrdiff_t>
 class _LIBCPP_TEMPLATE_VIS istream_iterator
+#if _LIBCPP_STD_VER <= 11 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
     : public iterator<input_iterator_tag, _Tp, _Distance, const _Tp*, const _Tp&>
+#endif
 {
 public:
     typedef _CharT char_type;
     typedef _Traits traits_type;
     typedef basic_istream<_CharT,_Traits> istream_type;
+    typedef input_iterator_tag iterator_category;
+    typedef _Tp value_type;
+    typedef _Distance difference_type;
+    typedef const _Tp* pointer;
+    typedef const _Tp& reference;
 private:
     istream_type* __in_stream_;
     _Tp __value_;
 public:
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR istream_iterator() : __in_stream_(nullptr), __value_() {}
     _LIBCPP_INLINE_VISIBILITY istream_iterator(istream_type& __s) : __in_stream_(_VSTD::addressof(__s))
         {
             if (!(*__in_stream_ >> __value_))

 operator!=(const istream_iterator<_Tp, _CharT, _Traits, _Distance>& __x,
            const istream_iterator<_Tp, _CharT, _Traits, _Distance>& __y)
 {
     return !(__x == __y);
 }
 
 template <class _Tp, class _CharT = char, class _Traits = char_traits<_CharT> >
 class _LIBCPP_TEMPLATE_VIS ostream_iterator
+#if _LIBCPP_STD_VER <= 11 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
     : public iterator<output_iterator_tag, void, void, void, void>
+#endif
 {
 public:
     typedef output_iterator_tag             iterator_category;
     typedef void                            value_type;
 #if _LIBCPP_STD_VER > 17
-    typedef std::ptrdiff_t                  difference_type;
+    typedef ptrdiff_t                       difference_type;
 #else
     typedef void                            difference_type;
 #endif
     typedef void                            pointer;
     typedef void                            reference;
     typedef _CharT                          char_type;
     typedef _Traits                         traits_type;
     typedef basic_ostream<_CharT, _Traits>  ostream_type;
 
 private:
     ostream_type* __out_stream_;
     const char_type* __delim_;
 public:
+#if _LIBCPP_STD_VER > 17
+    _LIBCPP_INLINE_VISIBILITY ostream_iterator() = default;
+#endif
     _LIBCPP_INLINE_VISIBILITY ostream_iterator(ostream_type& __s) _NOEXCEPT
         : __out_stream_(_VSTD::addressof(__s)), __delim_(nullptr) {}
     _LIBCPP_INLINE_VISIBILITY ostream_iterator(ostream_type& __s, const _CharT* __delimiter) _NOEXCEPT
         : __out_stream_(_VSTD::addressof(__s)), __delim_(__delimiter) {}
     _LIBCPP_INLINE_VISIBILITY ostream_iterator& operator=(const _Tp& __value_)
         {
             *__out_stream_ << __value_;
             if (__delim_)
                 *__out_stream_ << __delim_;
             return *this;
         }
 
     _LIBCPP_INLINE_VISIBILITY ostream_iterator& operator*()     {return *this;}
     _LIBCPP_INLINE_VISIBILITY ostream_iterator& operator++()    {return *this;}
     _LIBCPP_INLINE_VISIBILITY ostream_iterator& operator++(int) {return *this;}
 };
 
 template<class _CharT, class _Traits>
 class _LIBCPP_TEMPLATE_VIS istreambuf_iterator
+#if _LIBCPP_STD_VER <= 11 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
     : public iterator<input_iterator_tag, _CharT,
                       typename _Traits::off_type, _CharT*,
                       _CharT>
+#endif
 {
 public:
+    typedef input_iterator_tag              iterator_category;
+    typedef _CharT                          value_type;
+    typedef typename _Traits::off_type      difference_type;
+    typedef _CharT*                         pointer;
+    typedef _CharT                          reference;
     typedef _CharT                          char_type;
     typedef _Traits                         traits_type;
     typedef typename _Traits::int_type      int_type;
     typedef basic_streambuf<_CharT,_Traits> streambuf_type;
     typedef basic_istream<_CharT,_Traits>   istream_type;
 private:
     mutable streambuf_type* __sbuf_;
 

 template <class _CharT, class _Traits>
 inline _LIBCPP_INLINE_VISIBILITY
 bool operator!=(const istreambuf_iterator<_CharT,_Traits>& __a,
                 const istreambuf_iterator<_CharT,_Traits>& __b)
                 {return !__a.equal(__b);}
 
 template <class _CharT, class _Traits>
 class _LIBCPP_TEMPLATE_VIS ostreambuf_iterator
+#if _LIBCPP_STD_VER <= 11 || !defined(_LIBCPP_ABI_NO_ITERATOR_BASES)
     : public iterator<output_iterator_tag, void, void, void, void>
+#endif
 {
 public:
     typedef output_iterator_tag                 iterator_category;
     typedef void                                value_type;
 #if _LIBCPP_STD_VER > 17
-    typedef std::ptrdiff_t                      difference_type;
+    typedef ptrdiff_t                           difference_type;
 #else
     typedef void                                difference_type;
 #endif
     typedef void                                pointer;
     typedef void                                reference;
     typedef _CharT                              char_type;
     typedef _Traits                             traits_type;
     typedef basic_streambuf<_CharT, _Traits>    streambuf_type;
     typedef basic_ostream<_CharT, _Traits>      ostream_type;
 
 private:
     streambuf_type* __sbuf_;
 public:
+#if _LIBCPP_STD_VER > 17
+    _LIBCPP_INLINE_VISIBILITY ostreambuf_iterator() = default;
+#endif
     _LIBCPP_INLINE_VISIBILITY ostreambuf_iterator(ostream_type& __s) _NOEXCEPT
         : __sbuf_(__s.rdbuf()) {}
     _LIBCPP_INLINE_VISIBILITY ostreambuf_iterator(streambuf_type* __s) _NOEXCEPT
         : __sbuf_(__s) {}
     _LIBCPP_INLINE_VISIBILITY ostreambuf_iterator& operator=(_CharT __c)
         {
             if (__sbuf_ && traits_type::eq_int_type(__sbuf_->sputc(__c), traits_type::eof()))
                 __sbuf_ = nullptr;

libcxx/test/std/iterators/iterator.primitives/iterator.traits/cxx20_iterator_traits.compile.pass.cpp

 #include "test_iterators.h"
 #include "iterator_traits_cpp17_iterators.h"
 
 template <class Traits>
 constexpr bool has_iterator_concept_v = requires {
   typename Traits::iterator_concept;
 };
 
-template <class Iter, class Category, class Diff = void>
+template <class Iter, class Category>
 constexpr bool testIOIterator() {
   using Traits = std::iterator_traits<Iter>;
   static_assert(std::same_as<typename Traits::iterator_category, Category>);
   static_assert(std::same_as<typename Traits::value_type, void>);
-  static_assert(std::same_as<typename Traits::difference_type, Diff>);
+  static_assert(std::same_as<typename Traits::difference_type, std::ptrdiff_t>);
   static_assert(std::same_as<typename Traits::reference, void>);
   static_assert(std::same_as<typename Traits::pointer, void>);
   static_assert(!has_iterator_concept_v<Traits>);
 
   return true;
 }
 
 template <class Iter, class ValueType, class Category>

 static_assert(testIOIterator<std::back_insert_iterator<std::vector<int>>, std::output_iterator_tag>());
 static_assert(testIOIterator<std::front_insert_iterator<std::vector<int>>, std::output_iterator_tag>());
 static_assert(testIOIterator<std::insert_iterator<std::vector<int>>, std::output_iterator_tag>());
 static_assert(testConstWithoutConcept<std::istream_iterator<int, char>, int, std::input_iterator_tag>());
 
 #if !defined(_LIBCPP_HAS_NO_LOCALIZATION)
 static_assert(testWithoutConcept<std::istreambuf_iterator<char>, char, long long, char, char*, std::input_iterator_tag>());
 static_assert(testWithoutConcept<std::move_iterator<int*>, int, std::ptrdiff_t, int&&, int*, std::random_access_iterator_tag>());
-static_assert(testIOIterator<std::ostream_iterator<int, char>, std::output_iterator_tag, std::ptrdiff_t>());
-static_assert(testIOIterator<std::ostreambuf_iterator<int, char>, std::output_iterator_tag, std::ptrdiff_t>());
+static_assert(testIOIterator<std::ostream_iterator<int, char>, std::output_iterator_tag>());
+static_assert(testIOIterator<std::ostreambuf_iterator<int, char>, std::output_iterator_tag>());
 static_assert(testConstWithoutConcept<std::cregex_iterator, std::cmatch, std::forward_iterator_tag>());
 static_assert(testConstWithoutConcept<std::cregex_token_iterator, std::csub_match, std::forward_iterator_tag>());
 #endif // !_LIBCPP_HAS_NO_LOCALIZATION
 
 #ifndef _LIBCPP_HAS_NO_FILESYSTEM_LIBRARY
 static_assert(testWithoutConcept<std::filesystem::directory_iterator, std::filesystem::directory_entry, std::ptrdiff_t,
                                  const std::filesystem::directory_entry&, const std::filesystem::directory_entry*,
                                  std::input_iterator_tag>());

libcxx/test/std/iterators/predef.iterators/insert.iterators/back.insert.iterator/types.pass.cpp

 test()
 {
     typedef std::back_insert_iterator<C> R;
     C c;
     find_container<C> q(c);
     q.test();
     static_assert((std::is_same<typename R::container_type, C>::value), "");
     static_assert((std::is_same<typename R::value_type, void>::value), "");
+#if TEST_STD_VER <= 17
     static_assert((std::is_same<typename R::difference_type, void>::value), "");
+#else
+    static_assert((std::is_same<typename R::difference_type, std::ptrdiff_t>::value), "");
+#endif
     static_assert((std::is_same<typename R::reference, void>::value), "");
     static_assert((std::is_same<typename R::pointer, void>::value), "");
     static_assert((std::is_same<typename R::iterator_category, std::output_iterator_tag>::value), "");
 }
 
 int main(int, char**)
 {
     test<std::vector<int> >();
 
   return 0;
 }

libcxx/test/std/iterators/predef.iterators/insert.iterators/front.insert.iterator/types.pass.cpp

 test()
 {
     typedef std::front_insert_iterator<C> R;
     C c;
     find_container<C> q(c);
     q.test();
     static_assert((std::is_same<typename R::container_type, C>::value), "");
     static_assert((std::is_same<typename R::value_type, void>::value), "");
+#if TEST_STD_VER <= 17
     static_assert((std::is_same<typename R::difference_type, void>::value), "");
+#else
+    static_assert((std::is_same<typename R::difference_type, std::ptrdiff_t>::value), "");
+#endif
     static_assert((std::is_same<typename R::reference, void>::value), "");
     static_assert((std::is_same<typename R::pointer, void>::value), "");
     static_assert((std::is_same<typename R::iterator_category, std::output_iterator_tag>::value), "");
 }
 
 int main(int, char**)
 {
     test<std::vector<int> >();
 
   return 0;
 }

libcxx/test/std/iterators/predef.iterators/insert.iterators/insert.iterator/types.pass.cpp

 test()
 {
     typedef std::insert_iterator<C> R;
     C c;
     find_members<C> q(c);
     q.test();
     static_assert((std::is_same<typename R::container_type, C>::value), "");
     static_assert((std::is_same<typename R::value_type, void>::value), "");
+#if TEST_STD_VER <= 17
     static_assert((std::is_same<typename R::difference_type, void>::value), "");
+#else
+    static_assert((std::is_same<typename R::difference_type, std::ptrdiff_t>::value), "");
+#endif
     static_assert((std::is_same<typename R::reference, void>::value), "");
     static_assert((std::is_same<typename R::pointer, void>::value), "");
     static_assert((std::is_same<typename R::iterator_category, std::output_iterator_tag>::value), "");
 }
 
 int main(int, char**)
 {
     test<std::vector<int> >();
 
   return 0;
 }

libcxx/test/std/iterators/stream.iterators/ostream.iterator/iterator_concept_conformance.compile.pass.cpp

 
 #include <iterator>
 
 #include <iosfwd>
 
 using iterator = std::ostream_iterator<int, std::ostream>;
 static_assert(!std::indirectly_readable<iterator>);
 static_assert(std::indirectly_writable<iterator, int>);
-static_assert(!std::weakly_incrementable<iterator>);
-static_assert(!std::input_or_output_iterator<iterator>);
+static_assert(std::weakly_incrementable<iterator>);
+static_assert(std::input_or_output_iterator<iterator>);
 static_assert(!std::sentinel_for<iterator, iterator>);
 static_assert(!std::sized_sentinel_for<iterator, iterator>);
 static_assert(!std::input_iterator<iterator>);

libcxx/test/std/iterators/stream.iterators/ostream.iterator/ostream.iterator.cons.des/default.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <iterator>
+
+// class ostream_iterator
+
+// constexpr ostream_iterator() noexcept = default;
+
+#include <iterator>
+#include <string> // char_traits
+
+#include "test_macros.h"
+
+struct MyTraits : std::char_traits<char> {
+    MyTraits();  // This should not be called.
+};
+
+constexpr bool test_constexpr()
+{
+    std::ostream_iterator<int> it;
+    (void)it;
+    std::ostream_iterator<int, char, MyTraits> wit;
+    (void)wit;
+    return true;
+}
+
+int main(int, char**)
+{
+    ASSERT_NOEXCEPT(std::ostream_iterator<int>());
+    ASSERT_NOEXCEPT(std::ostream_iterator<int, char, MyTraits>());
+
+    test_constexpr();
+    static_assert(test_constexpr());
+
+    return 0;
+}

libcxx/test/std/iterators/stream.iterators/ostreambuf.iterator/iterator_concept_conformance.compile.pass.cpp

 #include <iterator>
 
 #include <ostream>
 #include <string>
 
 using iterator = std::ostreambuf_iterator<char>;
 static_assert(!std::indirectly_readable<iterator>);
 static_assert(std::indirectly_writable<iterator, char>);
-static_assert(!std::weakly_incrementable<iterator>);
-static_assert(!std::input_or_output_iterator<iterator>);
+static_assert(std::weakly_incrementable<iterator>);
+static_assert(std::input_or_output_iterator<iterator>);
 static_assert(!std::sentinel_for<iterator, iterator>);
 static_assert(!std::sized_sentinel_for<iterator, iterator>);
 static_assert(!std::input_iterator<iterator>);

libcxx/test/std/iterators/stream.iterators/ostreambuf.iterator/ostreambuf.iter.cons/default.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <iterator>
+
+// class ostreambuf_iterator
+
+// constexpr ostreambuf_iterator() noexcept = default;
+
+#include <iterator>
+
+#include "test_macros.h"
+
+constexpr bool test_constexpr()
+{
+    std::ostreambuf_iterator<char> it;
+    (void)it;
+    std::ostreambuf_iterator<wchar_t> wit;
+    (void)wit;
+    return true;
+}
+
+int main(int, char**)
+{
+    ASSERT_NOEXCEPT(std::ostreambuf_iterator<char>());
+    ASSERT_NOEXCEPT(std::ostreambuf_iterator<wchar_t>());
+
+    test_constexpr();
+    static_assert(test_constexpr());
+
+    return 0;
+}