Fix forward declaration of HardwareSerial for megaAVR architecture

[brusherru]

There is no issue.
The problem was reported on the forum: https://forum.xod.io/t/nano-every-upload-issue/4198

[evgenykochetkov]

The fixtures are not updated, but otherwise LGTM

[nkrkv]

I wonder why workspace/big-patch/__fixtures__/arduino.cpp has changed the node order (is it flanky?), but what’s about the PR itself:

LGTM

[nkrkv]

Oh, no, there’s a better way. Wait for a minute, I’ll explain. For now, I’m vetoing the merge

[nkrkv]

Take a look at the core used by Nano Every (https://github.com/arduino/ArduinoCore-megaavr). Namely, to the file https://github.com/arduino/ArduinoCore-megaavr/blob/master/cores/arduino/UART.h. See:

#include "api/HardwareSerial.h"

using namespace arduino;

What the api/. See README:

Copy or symlink the api folder from the ArduinoCore-API (https://github.com/arduino/ArduinoCore-API) repo.

Go there. Read:

This repository hosts the hardware independent layer of Arduino core.

All Arduino official cores are being ported to the new structure so they take advantage of this single repo.

Including this repo in your existing Arduino core will allow the language to grow and include new features. For backwards compatibility, every revision of this repo will increase ARDUINO_API_VERSION define.

Some cores have been ported to the new structure, for example: megaAVR, nRF52-mbedos, classic AVR, SAMD

Whoa! Some classic cores already use the new API wich has arduino namespace. So, merging this PR as is only fixes a little portion of the problem. It will return again and again as users update their packages.

So the proper way of fixing the issue would be using namespace arduino; at the global level. However, I afraid it’s too wide and can cause problems which are not apparent immediately. For now, a safer workaround would be looking at ARDUINO_API_VERSION macro definition, not ARDUINO_ARCH_MEGAAVR.

[brusherru]

@nkrkv done!

[nkrkv]

The fixtures do not match uart.h

[nkrkv]

LGTM