Added a deprecation warning for as<char*>()

extras/tests/Misc/CMakeLists.txt

@@ -5,6 +5,7 @@
 add_executable(MiscTests
 	arithmeticCompare.cpp
 	conflicts.cpp
+	deprecated.cpp
 	FloatParts.cpp
 	JsonString.cpp
 	printable.cpp

extras/tests/Misc/deprecated.cpp

@@ -0,0 +1,39 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright Benoit Blanchon 2014-2021
+// MIT License
+
+#include <ArduinoJson.h>
+#include <catch.hpp>
+
+#if defined(__GNUC__)
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+#endif
+
+#ifdef _MSC_VER
+#pragma warning(disable : 4996)  // deprecation warning
+#endif
+
+TEST_CASE("Deprecated features") {
+  StaticJsonDocument<256> doc;
+  const char* s = "hello";
+  doc["s"] = s;
+  doc["a"].add(s);
+
+  SECTION("JsonVariant::as<char*>()") {
+    JsonVariant v = doc["s"];
+    REQUIRE(v.as<char*>() == s);
+  }
+
+  SECTION("JsonVariantConst::as<char*>()") {
+    JsonVariantConst v = doc["s"];
+    REQUIRE(v.as<char*>() == s);
+  }
+
+  SECTION("MemberProxy::as<char*>()") {
+    REQUIRE(doc["s"].as<char*>() == s);
+  }
+
+  SECTION("ElementProxy::as<char*>()") {
+    REQUIRE(doc["a"][0].as<char*>() == s);
+  }
+}