error: invalid use of incomplete type InvalidConversion

InvalidConversion is a dummy class: it’s declared but not defined. ArduinoJson uses this class to trigger a recognizable compilation error.

This error occurs when you try to get a mutable reference from a read-only reference. For example, the following function receives a const reference to a JsonDocument, so it cannot get a JsonArray from it:

void printPorts(const JsonDocument& doc) {
  JsonArray ports = doc["ports"];  // <-- error here
  for (int port : ports)
    Serial.println(port);
}

If you try to compile this code, you’ll get an error like this one:

In file included from .../ArduinoJson/Variant/VariantRefBase.hpp:9:0,
                 from .../ArduinoJson/Array/ElementProxy.hpp:7,
                 from .../ArduinoJson/Array/JsonArray.hpp:7,
                 from .../ArduinoJson.hpp:29,
                 from .../ArduinoJson.h:9,
                 from MyProject.ino:1:
.../ArduinoJson/Variant/JsonVariantConst.hpp: In instantiation of 'typename ArduinoJson::V701L1::detail::enable_if<((! ArduinoJson::V701L1::detail::is_same<T, char*>::value) && (! ArduinoJson::V701L1::detail::is_same<T, char>::value)), T>::type ArduinoJson::V701L1::JsonVariantConst::as() const [with T = ArduinoJson::V701L1::JsonArray; typename ArduinoJson::V701L1::detail::enable_if<((! ArduinoJson::V701L1::detail::is_same<T, char*>::value) && (! ArduinoJson::V701L1::detail::is_same<T, char>::value)), T>::type = ArduinoJson::V701L1::JsonArray]':
.../ArduinoJson/Variant/JsonVariantConst.hpp:84:17:   required from 'ArduinoJson::V701L1::JsonVariantConst::operator T() const [with T = ArduinoJson::V701L1::JsonArray]'
MyProject.ino:4:32:   required from here
.../ArduinoJson/Variant/JsonVariantConst.hpp:69:34: error: invalid use of incomplete type 'class ArduinoJson::V701L1::detail::InvalidConversion<ArduinoJson::V701L1::JsonVariantConst, ArduinoJson::V701L1::JsonArray>'
     return Converter<T>::fromJson(*this);
            ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
In file included from .../ArduinoJson/Variant/VariantRefBase.hpp:8:0,
                 from .../ArduinoJson/Array/ElementProxy.hpp:7,
                 from .../ArduinoJson/Array/JsonArray.hpp:7,
                 from .../ArduinoJson.hpp:29,
                 from .../ArduinoJson.h:9,
                 from MyProject.ino:1:
.../ArduinoJson/Variant/Converter.hpp:20:7: note: declaration of 'class ArduinoJson::V701L1::detail::InvalidConversion<ArduinoJson::V701L1::JsonVariantConst, ArduinoJson::V701L1::JsonArray>'
 class InvalidConversion;  // Error here? See https://arduinojson.org/v7/invalid-conversion/
       ^~~~~~~~~~~~~~~~~

After removing the namespaces and all the remaining noise, you get this:

invalid use of incomplete type 'class InvalidConversion<JsonVariantConst, JsonArray>'

As you can see, the error message is quite explicit: you cannot convert a JsonVariantConst to JsonArray. Indeed, the const version of JsonDocument::operator[] returns a JsonVariantConst, which is read-only, so you cannot convert it to a mutable reference like JsonArray.

The solution is to replace the mutable reference type with the read-only equivalent. In this case, we must replace JsonArray with JsonArrayConst:

  void printPorts(const JsonDocument& doc) {
-   JsonArray ports = doc["ports"];
+   JsonArrayConst ports = doc["ports"];
    for (int port : ports)
      Serial.println(port);
  }