foo.cc:4:9: error: 'a' was not declared in this scope
    4 | int cmd(a,b)
      |         ^
foo.cc:4:11: error: 'b' was not declared in this scope
    4 | int cmd(a,b)
      |           ^
foo.cc:4:12: error: expression list treated as compound expression in initializer [-fpermissive]
    4 | int cmd(a,b)
      |            ^
foo.cc: In function 'int main()':
foo.cc:21:21: error: 'cmd' cannot be used as a function
   21 |  cout<<cmd(a,cmd(b,c));
      |                     ^
foo.cc:21:22: error: 'cmd' cannot be used as a function
   21 |  cout<<cmd(a,cmd(b,c));
      |                      ^

[Hydro](https://hydro.ac)提供评测服务