I’ve never had an Arduino and any blind code I post for it is just wishful thinking. 
So I depend on Arduino users’ feedback to know if it’s actually working! 
After all these years you’ve given me that feedback, thx! 
However, based on the big log you’ve posted above, I still think that’s rather a warning than an actual error. 
AFAIK, the C language is able to automatically fit in an unsigned char into a signed char b/c both are 8-bit datatypes. 
The log also mentions [-fpermissive], which I believe is a command line flag parameter which would allow the C compiler to ignore all those related warnings when compiling my code. 