invalid output constraint '=w' in asm

[voltangle]

So I tried to compile my app using Index target that is included with x-avr. One of the includes was avr/delay.h. I got an error about ASM:

invalid output constraint '=w' in asm

Here is also a screenshot:

But when I compile it with avr-g++(yes I am using c++) it compiles normally. What is the issue?

UPDATE: I probably figured it out. Index is probably using clang instead of avr-gcc, and that is the issue cause. Now another question, where can I find source code for Index?