JsonVariantConst bug with Arrays

[vladtepesch]

Describe the bug
JsonVariantConst behaves different than JsonVariant:

If assigning a JsonArray to JsonVariantConst .is<JsonArray>() gives false (while JsonVariant::is<JsonArray>() gives true)

Troubleshooter report
does not apply - question 3 and 4 does not map to problem

Environment
Here is the environment that I used:

• online compiler
• local gcc
• Microcontroller: ESP32-S3
• ArduinoJson-v7.4.3

Reproduction
Here is a small snippet that reproduces the issue.

https://onecompiler.com/cpp/44ffv6mub

#include <iostream>
#include "ArduinoJson.h"
int main() {
  JsonDocument doc;
  // Add an array
  JsonArray data = doc["data"].to<JsonArray>();
  data.add(127);
  data.add(255);
  data.add(0);
  
  JsonVariantConst varConst = data;
  JsonVariant      var      = data;

  std::cout <<"doc[\"data\"].is<JsonArray>(); " << doc["data"].is<JsonArray>() << std::endl;
  std::cout <<"varConst.is<JsonArray>(); " << varConst.is<JsonArray>() << std::endl;
  std::cout <<"var.is<JsonArray>(); " << var.is<JsonArray>() << std::endl;
}

Compiler output
n/a

Program output

Expected output:

doc["data"].is<JsonArray>(); 1
varConst.is<JsonArray>(); 1
var.is<JsonArray>(); 1

Actual output:

doc["data"].is<JsonArray>(); 1
varConst.is<JsonArray>(); 0    <-------
var.is<JsonArray>(); 1

[bblanchon]

Hi @vladtepesch,

Please apologize for the late answer.

JsonVariantConst being a read-only reference, it cannot be converted to a read-write reference such as JsonArray.
That's why JsonVariantConst::is<JsonArray>() always returns false.

If you want to test that a JsonVariantConst points to an array, you must use the read-only version of JsonArray: JsonArrayConst. For example:

if (varConst.is<JsonArrayConst>()) {
  // varConst points to an array
} 

The documentation did mention that JsonVariantConst::is<T>() always returns false for JsonArray, JsonObject, and JsonVariant, but it didn't explain why, so I added a dedicated paragraph:
https://arduinojson.org/v7/api/jsonvariantconst/is/

Best regards,
Benoit