Fix 'std::string' has not been declared (issue #1967)

Ported from 259855a

extras/tests/Misc/CMakeLists.txt

@@ -6,6 +6,7 @@ add_executable(MiscTests
 	arithmeticCompare.cpp
 	conflicts.cpp
 	FloatParts.cpp
+	issue1967.cpp
 	JsonString.cpp
 	NoArduinoHeader.cpp
 	printable.cpp

extras/tests/Misc/issue1967.cpp

@@ -0,0 +1,13 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2023, Benoit BLANCHON
+// MIT License
+
+// we expect ArduinoJson.h to include <string>
+#define ARDUINOJSON_ENABLE_STD_STRING 1
+
+// but we don't want it to included accidentally
+#undef ARDUINO
+#define ARDUINOJSON_ENABLE_STD_STREAM 0
+#define ARDUINOJSON_ENABLE_STRING_VIEW 0
+
+#include <ArduinoJson.h>

src/ArduinoJson/Variant/ConverterImpl.hpp

@@ -9,6 +9,10 @@
 #include <ArduinoJson/Polyfills/utility.hpp>
 #include <ArduinoJson/Variant/JsonVariantConst.hpp>
 
+#if ARDUINOJSON_ENABLE_STD_STRING
+#  include <string>
+#endif
+
 ARDUINOJSON_BEGIN_PUBLIC_NAMESPACE
 
 template <typename T, typename Enable>