Oh, that must be a typo in the document, sorry. I just fixed it. The actual corodemo.cc file was correct, though: http://www.scs.stanford.edu/~dm/blog/corodemo.cc

Also, here's a more authoritative source than stack overflow for the return type of main: https://timsong-cpp.github.io/cppwp/n4861/basic.start.main#2