working on #235

doc/examples/push_back__initializer_list.cpp

+#include <json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create JSON values
+    json object = {{"one", 1}, {"two", 2}};
+    json null;
+
+    // print values
+    std::cout << object << '\n';
+    std::cout << null << '\n';
+
+    // add values:
+    object.push_back({"three", 3});  // object is extended
+    object += {"four", 4};           // object is extended
+    null.push_back({"five", 5});     // null is converted to array
+
+    // print values
+    std::cout << object << '\n';
+    std::cout << null << '\n';
+
+    // would throw:
+    //object.push_back({1, 2, 3});
+}

doc/examples/push_back__initializer_list.link

+<a target="_blank" href="http://melpon.org/wandbox/permlink/wZF4dRHjfCyjb3rx"><b>online</b></a>
\ No newline at end of file

doc/examples/push_back__initializer_list.output

+{"one":1,"two":2}
+null
+{"four":4,"one":1,"three":3,"two":2}
+[["five",5]]

src/json.hpp

@@ -4878,7 +4878,55 @@ class basic_json
     reference operator+=(const typename object_t::value_type& val)
     {
         push_back(val);
-        return operator[](val.first);
+        return *this;
+    }
+
+    /*!
+    @brief add an object to an object
+
+    This function allows to use `push_back` with an initializer list. In case
+
+    1. the current value is an object,
+    2. the initializer list @a init contains only two elements, and
+    3. the first element of @a init is a string,
+
+    @a init is converted into an object element and added using
+    @ref push_back(const typename object_t::value_type&). Otherwise, @a init
+    is converted to a JSON value and added using @ref push_back(basic_json&&).
+
+    @param init  an initializer list
+
+    @complexity Linear in the size of the initializer list @a init.
+
+    @note This function is required to resolve an ambiguous overload error,
+          because pairs like `{"key", "value"}` can be both interpreted as
+          `object_t::value_type` or `std::initializer_list<basic_json>`, see
+          https://github.com/nlohmann/json/issues/235 for more information.
+
+    @liveexample{The example shows how initializer lists are treated as
+    objects when possible.,push_back__initializer_list}
+    */
+    void push_back(std::initializer_list<basic_json> init)
+    {
+        if (is_object() and init.size() == 2 and init.begin()->is_string())
+        {
+            const string_t key = *init.begin();
+            push_back(typename object_t::value_type(key, *(init.begin() + 1)));
+        }
+        else
+        {
+            push_back(basic_json(init));
+        }
+    }
+
+    /*!
+    @brief add an object to an object
+    @copydoc push_back(std::initializer_list<basic_json>)
+    */
+    reference operator+=(std::initializer_list<basic_json> init)
+    {
+        push_back(init);
+        return *this;
     }
 
     /*!

src/json.hpp.re2c

@@ -4878,7 +4878,55 @@ class basic_json
     reference operator+=(const typename object_t::value_type& val)
     {
         push_back(val);
-        return operator[](val.first);
+        return *this;
+    }
+
+    /*!
+    @brief add an object to an object
+
+    This function allows to use `push_back` with an initializer list. In case
+
+    1. the current value is an object,
+    2. the initializer list @a init contains only two elements, and
+    3. the first element of @a init is a string,
+
+    @a init is converted into an object element and added using
+    @ref push_back(const typename object_t::value_type&). Otherwise, @a init
+    is converted to a JSON value and added using @ref push_back(basic_json&&).
+
+    @param init  an initializer list
+
+    @complexity Linear in the size of the initializer list @a init.
+
+    @note This function is required to resolve an ambiguous overload error,
+          because pairs like `{"key", "value"}` can be both interpreted as
+          `object_t::value_type` or `std::initializer_list<basic_json>`, see
+          https://github.com/nlohmann/json/issues/235 for more information.
+
+    @liveexample{The example shows how initializer lists are treated as
+    objects when possible.,push_back__initializer_list}
+    */
+    void push_back(std::initializer_list<basic_json> init)
+    {
+        if (is_object() and init.size() == 2 and init.begin()->is_string())
+        {
+            const string_t key = *init.begin();
+            push_back(typename object_t::value_type(key, *(init.begin() + 1)));
+        }
+        else
+        {
+            push_back(basic_json(init));
+        }
+    }
+
+    /*!
+    @brief add an object to an object
+    @copydoc push_back(std::initializer_list<basic_json>)
+    */
+    reference operator+=(std::initializer_list<basic_json> init)
+    {
+        push_back(init);
+        return *this;
     }
 
     /*!

test/unit.cpp

@@ -7920,6 +7920,42 @@ TEST_CASE("modifiers")
                                   "cannot use push_back() with number");
             }
         }
+
+        SECTION("with initializer_list")
+        {
+            SECTION("null")
+            {
+                json j;
+                j.push_back({"foo", "bar"});
+                CHECK(j == json::array({{"foo", "bar"}}));
+
+                json k;
+                k.push_back({1, 2, 3});
+                CHECK(k == json::array({{1, 2, 3}}));
+            }
+
+            SECTION("array")
+            {
+                json j = {1, 2, 3};
+                j.push_back({"foo", "bar"});
+                CHECK(j == json({1, 2, 3, {"foo", "bar"}}));
+
+                json k = {1, 2, 3};
+                k.push_back({1, 2, 3});
+                CHECK(k == json({1, 2, 3, {1, 2, 3}}));
+            }
+
+            SECTION("object")
+            {
+                json j = {{"key1", 1}};
+                j.push_back({"key2", "bar"});
+                CHECK(j == json({{"key1", 1}, {"key2", "bar"}}));
+
+                json k = {{"key1", 1}};
+                CHECK_THROWS_AS(k.push_back({1, 2, 3, 4}), std::domain_error);
+                CHECK_THROWS_WITH(k.push_back({1, 2, 3, 4}), "cannot use push_back() with object");
+            }
+        }
     }
 
     SECTION("operator+=")
@@ -8016,6 +8052,42 @@ TEST_CASE("modifiers")
                                   "cannot use push_back() with number");
             }
         }
+
+        SECTION("with initializer_list")
+        {
+            SECTION("null")
+            {
+                json j;
+                j += {"foo", "bar"};
+                CHECK(j == json::array({{"foo", "bar"}}));
+
+                json k;
+                k += {1, 2, 3};
+                CHECK(k == json::array({{1, 2, 3}}));
+            }
+
+            SECTION("array")
+            {
+                json j = {1, 2, 3};
+                j += {"foo", "bar"};
+                CHECK(j == json({1, 2, 3, {"foo", "bar"}}));
+
+                json k = {1, 2, 3};
+                k += {1, 2, 3};
+                CHECK(k == json({1, 2, 3, {1, 2, 3}}));
+            }
+
+            SECTION("object")
+            {
+                json j = {{"key1", 1}};
+                j += {"key2", "bar"};
+                CHECK(j == json({{"key1", 1}, {"key2", "bar"}}));
+
+                json k = {{"key1", 1}};
+                CHECK_THROWS_AS((k += {1, 2, 3, 4}), std::domain_error);
+                CHECK_THROWS_WITH((k += {1, 2, 3, 4}), "cannot use push_back() with object");
+            }
+        }
     }
 
     SECTION("insert")
@@ -13992,6 +14064,15 @@ TEST_CASE("regression tests")
 
         CHECK(dest == expected);
     }
+
+    SECTION("issue ##235 - ambiguous overload for 'push_back' and 'operator+='")
+    {
+        json data = {{"key", "value"}};
+        data.push_back({"key2", "value2"});
+        data += {"key3", "value3"};
+
+        CHECK(data == json({{"key", "value"}, {"key2", "value2"}, {"key3", "value3"}}));
+    }
 }
 
 // special test case to check if memory is leaked if constructor throws