Changed the array subscript to automatically add missing elements

extras/tests/ElementProxy/set.cpp

@@ -9,7 +9,6 @@ using namespace ARDUINOJSON_NAMESPACE;
 
 TEST_CASE("ElementProxy::set()") {
   DynamicJsonDocument doc(4096);
-  doc.addElement();
   ElementProxy<JsonDocument&> ep = doc[0];
 
   SECTION("set(int)") {

extras/tests/ElementProxy/subscript.cpp

@@ -0,0 +1,25 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2020
+// MIT License
+
+#include <ArduinoJson.h>
+#include <catch.hpp>
+
+using namespace ARDUINOJSON_NAMESPACE;
+
+TEST_CASE("MemberProxy::operator[]") {
+  DynamicJsonDocument doc(4096);
+  ElementProxy<JsonDocument&> ep = doc[1];
+
+  SECTION("set member") {
+    ep["world"] = 42;
+
+    REQUIRE(doc.as<std::string>() == "[null,{\"world\":42}]");
+  }
+
+  SECTION("set element") {
+    ep[2] = 42;
+
+    REQUIRE(doc.as<std::string>() == "[null,[null,null,42]]");
+  }
+}