Using auto type to access array nested in object crashes device

[prototypicalpro]

Description
Running the following code snippet works great:

StaticJsonDocument<500> doc;
constexpr char str[] = "{\"contents\":[{\"module\":\"Packet\"},{\"module\":\"Analog\"}]}";
deserializeJson(doc, str);
JsonObject sensor = thing["contents"][1];
Serial.println(sensor.isNull()); // <- prints "0"

however, changing the type to auto causes my device to hang:

StaticJsonDocument<500> doc;
constexpr char str[] = "{\"contents\":[{\"module\":\"Packet\"},{\"module\":\"Analog\"}]}";
deserializeJson(doc, str);
auto sensor = thing["contents"][1];
Serial.println(sensor.isNull()); // <- Never printed, and must be reset to continue

I suspect this issue is due to ElementProxy type not resolving properly, but am unsure why this would be causing the device to crash.

Platform
I am using ArduinoJson 6.12.0, Arduino IDE 1.8.10, and running this sketch on the Adafruit Feather M0 (SAMD21) with Adafruit SAMD Core 1.5.2 . I've attached the
relevant sketch, as well as the build log.

[ewaldc]

@prototypicalpro,

auto sensor = thing["contents"][1];
decltype(sensor) b;

src\main.cpp:15:20: error: no matching function for call to 'ArduinoJson6120_000001::ElementProxy<const ArduinoJson6120_000001::MemberProxy<const ArduinoJson6120_000001::ObjectRef&, const char*>&>::ElementProxy()'
decltype(sensor) b;

So indeed it resolves to class ElementProxy, which overlays the = operator, so the problem could be there somewhere.

PS the code also crashes/resets an ESP8266 (and other devices)

[bblanchon]

Hi @prototypicalpro,

Thank you for reporting this bug.
The fix is available in the "6.x" branch.

Best Regards,
Benoit

[bblanchon]

Fix published in ArduinoJson 6.13.0