/in/foo.cc:1:0: warning: ignoring #pragma optimize  [-Wunknown-pragmas]
 %:pragma optimize(3)
 
/usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status