[libc++] Implements LWG3600 Making istream_iterator copy constructor trivial is an ABI break (#127343)

Closes: #105003

Author: mordante

libcxx/docs/Status/Cxx23Issues.csv

@@ -194,7 +194,7 @@
 "`LWG3569 <https://wg21.link/LWG3569>`__","``join_view`` fails to support ranges of ranges with non-default_initializable iterators","2022-11 (Kona)","","",""
 "`LWG3594 <https://wg21.link/LWG3594>`__","``inout_ptr`` — inconsistent ``release()`` in destructor","2022-11 (Kona)","|Complete|","19",""
 "`LWG3597 <https://wg21.link/LWG3597>`__","Unsigned integer types don't model advanceable","2022-11 (Kona)","","",""
-"`LWG3600 <https://wg21.link/LWG3600>`__","Making ``istream_iterator`` copy constructor trivial is an ABI break","2022-11 (Kona)","","",""
+"`LWG3600 <https://wg21.link/LWG3600>`__","Making ``istream_iterator`` copy constructor trivial is an ABI break","2022-11 (Kona)","|Nothing To Do|","",""
 "`LWG3629 <https://wg21.link/LWG3629>`__","``make_error_code`` and ``make_error_condition`` are customization points","2022-11 (Kona)","|Complete|","16",""
 "`LWG3636 <https://wg21.link/LWG3636>`__","``formatter<T>::format`` should be ``const``-qualified","2022-11 (Kona)","|Complete|","16",""
 "`LWG3646 <https://wg21.link/LWG3646>`__","``std::ranges::view_interface::size`` returns a signed type","2022-11 (Kona)","|Complete|","16",""

libcxx/include/__iterator/istream_iterator.h

@@ -58,6 +58,9 @@ class _LIBCPP_TEMPLATE_VIS istream_iterator
       __in_stream_ = nullptr;
   }
 
+  // LWG3600 Changed the wording of the copy constructor. In libc++ this constructor
+  // can still be trivial after this change.
+
   _LIBCPP_HIDE_FROM_ABI const _Tp& operator*() const { return __value_; }
   _LIBCPP_HIDE_FROM_ABI const _Tp* operator->() const { return std::addressof((operator*())); }
   _LIBCPP_HIDE_FROM_ABI istream_iterator& operator++() {

libcxx/include/iterator

@@ -530,7 +530,7 @@ public:
     istream_iterator(); // constexpr since C++11
     constexpr istream_iterator(default_sentinel_t); // since C++20
     istream_iterator(istream_type& s);
-    istream_iterator(const istream_iterator& x);
+    constexpr istream_iterator(const istream_iterator& x) noexcept(see below);
     ~istream_iterator();
 
     const T& operator*() const;

libcxx/test/std/iterators/stream.iterators/istream.iterator/istream.iterator.cons/copy.pass.cpp

@@ -10,22 +10,45 @@
 
 // class istream_iterator
 
-// istream_iterator(const istream_iterator& x);
-//  C++17 says:  If is_trivially_copy_constructible_v<T> is true, then
-//     this constructor is a trivial copy constructor.
+// istream_iterator(const istream_iterator& x) noexcept(see below);
 
 #include <iterator>
 #include <sstream>
 #include <cassert>
 
 #include "test_macros.h"
 
+// The copy constructor is constexpr in C++11, but that is not easy to test.
+// The comparison of the class is not constexpr so this is only a compile test.
+TEST_CONSTEXPR_CXX14 bool test_constexpr() {
+  std::istream_iterator<int> io;
+  [[maybe_unused]] std::istream_iterator<int> i = io;
+
+  return true;
+}
+
+struct thowing_copy_constructor {
+  thowing_copy_constructor() {}
+  thowing_copy_constructor(const thowing_copy_constructor&) TEST_NOEXCEPT_FALSE {}
+};
+
 int main(int, char**)
 {
     {
         std::istream_iterator<int> io;
         std::istream_iterator<int> i = io;
         assert(i == std::istream_iterator<int>());
+#if TEST_STD_VER >= 11
+        static_assert(std::is_nothrow_copy_constructible<std::istream_iterator<int>>::value, "");
+#endif
+    }
+    {
+      std::istream_iterator<thowing_copy_constructor> io;
+      std::istream_iterator<thowing_copy_constructor> i = io;
+      assert(i == std::istream_iterator<thowing_copy_constructor>());
+#if TEST_STD_VER >= 11
+      static_assert(!std::is_nothrow_copy_constructible<std::istream_iterator<thowing_copy_constructor>>::value, "");
+#endif
     }
     {
         std::istringstream inf(" 1 23");
@@ -37,5 +60,9 @@ int main(int, char**)
         assert(j == 1);
     }
 
-  return 0;
+#if TEST_STD_VER >= 14
+    static_assert(test_constexpr(), "");
+#endif
+
+    return 0;
 }