Copy values before inserting an initializer list into an array

insert(pos, {...}) inserted wrong values when the initializer list
contained const references to elements of the array being inserted
into. json_ref stores only a pointer for a const lvalue, so the
initializer_list_t range passed straight to the array's range insert
aliased the array's own storage; std::vector::insert(pos, first, last)
may move or shift elements before copying from that range, so the
source elements were already stale by the time they were read
(different wrong results on libc++ and libstdc++).

Copy the referenced values into a temporary array_t first, then move
that temporary into place, so the source range never aliases the
array being modified.

Fixes #5656.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/json.hpp

@@ -4152,8 +4152,16 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             JSON_THROW(invalid_iterator::create(202, "iterator does not fit current value", this));
         }
 
+        // copy the values first: ilist may refer to elements of this array
+        array_t values;
+        values.reserve(ilist.size());
+        for (const auto& element : ilist)
+        {
+            values.push_back(element.moved_or_copied());
+        }
+
         // insert to array and return iterator
-        return insert_iterator(pos, ilist.begin(), ilist.end());
+        return insert_iterator(pos, std::make_move_iterator(values.begin()), std::make_move_iterator(values.end()));
     }
 
     /// @brief inserts range of elements into object