Unfortunately __has_include(<ArduinoRS485.h>) will evaluate as false even when the library is installed. the reason is that __has_include checks whether the file is present in the compiler's "search path". Libraries are added to the search path by the "library discovery" phase of the Arduino sketch build system, which discovers libraries based on the #include directives in the code. So there is a "Catch-22" situation when attempting to use __has_include for conditional inclusion of headers from libraries:

arduino/arduino-cli#1650

You can check this with the following code:

#if __has_include(<ArduinoRS485.h>)
#pragma message "__has_include(<ArduinoRS485.h>) evaluated to true"
#else
#pragma message "__has_include(<ArduinoRS485.h>) evaluated to false"
#endif

When you compile, you will see the "__has_include(<ArduinoRS485.h>) evaluated to false" message in the output even when the ArduinoRS485 library is installed.

The above is true for Arduino CLI and Arduino IDE. I don't have any experience with Arduino PLC IDE, so I can't say for sure but I would assume it is also true there.

A workaround would be to add instructions for the user that, if they want to use the RS-485 debugging capabilities, they must add an #include directive for ArduinoRS485.h to their sketch above the #include directive for the Arduino_UnifiedStorage library:

#include <ArduinoRS485.h>
#include <Arduino_UnifiedStorage.h>

This will cause the ArduinoRS485 library to be discovered and its path added to the search path before the evaluation of __has_include(<ArduinoRS485.h>), which will cause it to evaluate as true.

@per1234 Well spotted! Your suggestion makes sense. Either we go with that, or we add ArduinoRS485 to the dependencies. I'd prefer not to because this dependency doesn't make sense for all boards. Wdyt?

I'd prefer not to because this dependency doesn't make sense for all boards. Wdyt?

Since the library's "debugging output" feature is completely supplemental and likely to be used by advanced users who won't have difficulty installing the ArduinoRS485 library on demand, I think your perspective regarding avoiding the introduction of an additional dependency is reasonable.