Is it possible to call return in a synchronized block ?

template
int SafeRingBufferN::read_char() {
synchronized {
return RingBufferN::read_char();
}
}

->
template
int SafeRingBufferN::read_char() {
int ret = 0;
synchronized {
ret = RingBufferN::read_char();
}
return ret;
}

At least this fix the compilation warning

Arduino15\packages\arduino\hardware\samd\1.8.11\cores\arduino\USB\SAMD21_USBDevice.h
uint32_t len = 0;
synchronized {
len = _rx_buffer.availableForStore();
}