My source code has typos by accident ... (1<<ADTS2,1<<ADTS1,1<<ADTS0).
Both ',' have to be '|'.

With compiler options   -Wall -pedantic   the warning

1

"warning: left operand of comma operator has no effect [-Wunused-value]"

should be returned by the compiler.

.

1

>>> avr-g++ -c -mmcu=atmega128rfa1 -pedantic -Wall -o weg.o weg.c

OK ... compiler warns
.

1

>>> avr-gcc -c -mmcu=atmega128rfa1 -pedantic -Wall -o weg.o weg.c

OK ... compiler warns
.

1

>>> avr-g++ -c -mmcu=atmega128rfa1 -pedantic -Wall -DAVR_IO=1 -o weg.o weg.c

NOT_OK ... avr-g+ hides  the warning with the "avr/io.h" included.
It has been tested with avr-g++ (GCC) 4.9.2 by myself and a
fellow has checked GCC 5.4.0.
.

1

>>> avr-gcc -c -mmcu=atmega128rfa1 -pedantic -Wall -DAVR_IO=1 -o weg.o weg.c

OK ... compiler warns
.

Attached files
- "weg.c" is the source code for test (commands are added as comments, 
-DAVR_IO=1 includes "avr/io.h")
- "weg.lis" the command line output (cmd.exe @ Win7)


I would like to know what allows the compiler to hide the warnings ... 
or what is the reason for not having the warning in this special 
context.