Error(s):

source_file.cpp: In function ‘int factorial(int&)’:
source_file.cpp:10:24: error: invalid initialization of non-const reference of type ‘int&’ from an rvalue of type ‘int’
source_file.cpp:7:5: error: in passing argument 1 of ‘int factorial(int&)’
source_file.cpp:11:1: warning: control reaches end of non-void function [-Wreturn-type]