Hi, This is great news. The explicitusing statements seem to provide backwards compatibility with existing forward declarations. As noted above, I had to add theusing arduino::String statement to make the fix more complete.

I validated your 2 PRs,

• arduino/ArduinoCore-samd#612
• #144

by switching my git repos to your branches (so basically, I get a samd 1.8.11, plus your changes). I then compiled and uploaded about 70 of my unit tests, across most of my libraries, to an actual MKR1000 that I recently purchased. I had a few hiccups, but eventually got everything working.

There were 3 classes of problems:

1. String forward declarations, fixed by above.
2. More stringent compiler errors and warnings from the SAMD21 compiler. (Every compiler seems to complain about slightly different things...)
3. The breaking change to::digitalWrite(uint8_t, uint8_t) which is now::digitalWrite(uint8_t, PinStatus) in the new API.
   • I have to use an explicitly scoped::digitalWrite() because I have an overloadeddigitalWrite() in that class. But::digitalWrite() does not match yourarduino::digitalWrite()) overload. So I had to insert a handful of#if defined(ARDUINO_API_VERSION) statements.

With regards to the 2 PR on my libraries:

• bxparks/AceTime#57
• bxparks/AceCommon#14

I will check in a version that removes that#error for the SAMD21 core. But I want to retain the blacklist for megaAVR platform until I personally verify thatarduino/ArduinoCore-megaavr#62 is fixed on actual hardware. I currently don't have any megaAVR boards, but it's on my shopping list.

I have a question onbxparks/AceTime#57 that you sent: You replaced the#include <Arduino.h> with an#include <arvr/pgmspace.h>. Unfortunately, that breaks the ESP32 platform, where it needs to be#include <pgmspace.h>. I can add some conditional code onARDUINO_API_VERSION in there, or just keep using#include <Arduino.h> in that.cpp file.

I will update this comment when I have submitted the fixes on my end that adds support for arduino:samd >= 1.8.10.