Override n + iterator operator in the iterator

src/json.hpp

@@ -8474,18 +8474,29 @@ class basic_json
         @brief  add to iterator
         @pre The iterator is initialized; i.e. `m_object != nullptr`.
         */
-        iter_impl operator+(difference_type i)
+        iter_impl operator+(difference_type i) const
         {
             auto result = *this;
             result += i;
             return result;
         }
 
+        /*!
+        @brief  addition of distance and iterator
+        @pre The iterator is initialized; i.e. `m_object != nullptr`.
+        */
+        friend iter_impl operator+(difference_type i, const iter_impl& it)
+        {
+            auto result = it;
+            result += i;
+            return result;
+        }
+
         /*!
         @brief  subtract from iterator
         @pre The iterator is initialized; i.e. `m_object != nullptr`.
         */
-        iter_impl operator-(difference_type i)
+        iter_impl operator-(difference_type i) const
         {
             auto result = *this;
             result -= i;

test/src/unit-iterators2.cpp

@@ -269,6 +269,16 @@ TEST_CASE("iterators 2")
                     CHECK_THROWS_AS(it + 1, json::invalid_iterator);
                     CHECK_THROWS_WITH(it + 1, "[json.exception.invalid_iterator.209] cannot use offsets with object iterators");
                 }
+                {
+                    auto it = j_object.begin();
+                    CHECK_THROWS_AS(1 + it, json::invalid_iterator);
+                    CHECK_THROWS_WITH(1 + it, "[json.exception.invalid_iterator.209] cannot use offsets with object iterators");
+                }
+                {
+                    auto it = j_object.cbegin();
+                    CHECK_THROWS_AS(1 + it, json::invalid_iterator);
+                    CHECK_THROWS_WITH(1 + it, "[json.exception.invalid_iterator.209] cannot use offsets with object iterators");
+                }
                 {
                     auto it = j_object.begin();
                     CHECK_THROWS_AS(it -= 1, json::invalid_iterator);
@@ -688,6 +698,16 @@ TEST_CASE("iterators 2")
                     CHECK_THROWS_AS(it + 1, json::invalid_iterator);
                     CHECK_THROWS_WITH(it + 1, "[json.exception.invalid_iterator.209] cannot use offsets with object iterators");
                 }
+                {
+                    auto it = j_object.rbegin();
+                    CHECK_THROWS_AS(1 + it, json::invalid_iterator);
+                    CHECK_THROWS_WITH(1 + it, "[json.exception.invalid_iterator.209] cannot use offsets with object iterators");
+                }
+                {
+                    auto it = j_object.crbegin();
+                    CHECK_THROWS_AS(1 + it, json::invalid_iterator);
+                    CHECK_THROWS_WITH(1 + it, "[json.exception.invalid_iterator.209] cannot use offsets with object iterators");
+                }
                 {
                     auto it = j_object.rbegin();
                     CHECK_THROWS_AS(it -= 1, json::invalid_iterator);