[libc++] Fix assignment in insertion into vector (#116001)

Changes:
- Avoid direct assignment in iterator-pair `insert` overload and
`insert_range`, except when the assignment is move assignment.

Author: frederick-vs-ja

libcxx/include/__vector/vector.h

@@ -58,6 +58,7 @@
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_trivially_relocatable.h>
 #include <__type_traits/type_identity.h>
+#include <__utility/declval.h>
 #include <__utility/exception_guard.h>
 #include <__utility/forward.h>
 #include <__utility/is_pointer_in_range.h>
@@ -602,6 +603,30 @@ class _LIBCPP_TEMPLATE_VIS vector {
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void
   __assign_with_size(_Iterator __first, _Sentinel __last, difference_type __n);
 
+  template <class _Iterator,
+            __enable_if_t<!is_same<decltype(*std::declval<_Iterator&>())&&, value_type&&>::value, int> = 0>
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void
+  __insert_assign_n_unchecked(_Iterator __first, difference_type __n, pointer __position) {
+    for (pointer __end_position = __position + __n; __position != __end_position; ++__position, (void)++__first) {
+      __temp_value<value_type, _Allocator> __tmp(this->__alloc_, *__first);
+      *__position = std::move(__tmp.get());
+    }
+  }
+
+  template <class _Iterator,
+            __enable_if_t<is_same<decltype(*std::declval<_Iterator&>())&&, value_type&&>::value, int> = 0>
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void
+  __insert_assign_n_unchecked(_Iterator __first, difference_type __n, pointer __position) {
+#if _LIBCPP_STD_VER >= 23
+    if constexpr (!forward_iterator<_Iterator>) { // Handles input-only sized ranges for insert_range
+      ranges::copy_n(std::move(__first), __n, __position);
+    } else
+#endif
+    {
+      std::copy_n(__first, __n, __position);
+    }
+  }
+
   template <class _InputIterator, class _Sentinel>
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI iterator
   __insert_with_sentinel(const_iterator __position, _InputIterator __first, _Sentinel __last);
@@ -1322,19 +1347,12 @@ vector<_Tp, _Allocator>::__insert_with_size(
           __construct_at_end(__m, __last, __n - __dx);
           if (__dx > 0) {
             __move_range(__p, __old_last, __p + __n);
-            std::copy(__first, __m, __p);
+            __insert_assign_n_unchecked(__first, __dx, __p);
           }
         }
       } else {
         __move_range(__p, __old_last, __p + __n);
-#if _LIBCPP_STD_VER >= 23
-        if constexpr (!forward_iterator<_Iterator>) {
-          ranges::copy_n(std::move(__first), __n, __p);
-        } else
-#endif
-        {
-          std::copy_n(__first, __n, __p);
-        }
+        __insert_assign_n_unchecked(std::move(__first), __n, __p);
       }
     } else {
       __split_buffer<value_type, allocator_type&> __v(__recommend(size() + __n), __p - this->__begin_, this->__alloc_);

libcxx/test/std/containers/sequences/vector/vector.modifiers/insert_iter_iter_iter.pass.cpp

@@ -11,6 +11,8 @@
 // template <class Iter>
 //   iterator insert(const_iterator position, Iter first, Iter last);
 
+// XFAIL: FROZEN-CXX03-HEADERS-FIXME
+
 #include <vector>
 #include <cassert>
 #include <cstddef>
@@ -161,6 +163,24 @@ TEST_CONSTEXPR_CXX20 bool tests() {
     for (; j < 105; ++j)
       assert(v[j] == 0);
   }
+  { // Ensure that iterator-pair insert() doesn't use unexpected assignment.
+    struct Wrapper {
+      TEST_CONSTEXPR Wrapper(int n) : n_(n) {}
+
+      int n_;
+
+    private:
+      void operator=(int);
+    };
+
+    int a[]                 = {1, 2, 3, 4, 5};
+    const std::size_t count = sizeof(a) / sizeof(a[0]);
+    std::vector<Wrapper> v;
+    v.insert(v.end(), a, a + count);
+    assert(v.size() == count);
+    for (std::size_t i = 0; i != count; ++i)
+      assert(v[i].n_ == a[i]);
+  }
 #if TEST_STD_VER >= 11
   {
     typedef std::vector<int, min_allocator<int> > V;

libcxx/test/std/containers/sequences/vector/vector.modifiers/insert_range.pass.cpp

@@ -64,6 +64,22 @@ constexpr bool test() {
     }
   }
 
+  { // Ensure that insert_range doesn't use unexpected assignment.
+    struct Wrapper {
+      constexpr Wrapper(int n) : n_(n) {}
+      void operator=(int) = delete;
+
+      int n_;
+    };
+
+    int a[]{1, 2, 3, 4, 5};
+    std::vector<Wrapper> v;
+    v.insert_range(v.end(), a);
+    assert(v.size() == std::size(a));
+    for (std::size_t i = 0; i != std::size(a); ++i)
+      assert(v[i].n_ == a[i]);
+  }
+
   return true;
 }