Add meta function ConverterNeedsWriteableRef<T>

src/ArduinoJson/Variant/ConverterImpl.hpp

@@ -303,4 +303,18 @@ inline bool canConvertFromJson(VariantConstRef src, const std::string_view&) {
 
 #endif
 
+template <typename T>
+struct ConverterNeedsWriteableRef {
+ protected:  // <- to avoid GCC's "all member functions in class are private"
+  typedef char Yes[1];
+  typedef char No[2];
+
+  static Yes& probe(T (*f)(VariantRef));
+  static No& probe(T (*f)(VariantConstRef));
+
+ public:
+  static const bool value =
+      sizeof(probe(Converter<T>::fromJson)) == sizeof(Yes);
+};
+
 }  // namespace ARDUINOJSON_NAMESPACE