What will this C code do?

void main(){
int x=1;
printf("%d\n",x=2);
}

>>129
It will be ignored. There won't be any trace of it after the program is compiled. However, if you're trying to use it, the compiler will obviously complain. Since you're so fond of the standard:
If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof operator whose result is an integer constant), somewhere in the entire program there shall be exactly one external definition for the identifier; otherwise, there shall be no more than one.