gistfile1.txt

body common control {
        bundlesequence => {
                base,
                g,
        };

        inputs => {
                "cfengine_stdlib.cf",
        };
}


bundle agent base {
vars:
        "sys" slist => {
                "A", "B", "C"
        };

commands:
        "/bin/true"
                classes => if_ok("${sys}");

reports:
        linux::
        "setting up ${sys}";
}

bundle agent g {
vars:
        A::
                "fok[1]" string => "foka";


        B::
                "fok[2]" string => "fokb";


        C::
                "fok[3]" string => "fokc";

        any::
                "fok_idx" slist => getindices("fok");
reports:
        linux::
                "foks: ${fok_idx}";
                "${fok[1]}";
}

# cf-agent -f test.cf result:
# R: setting up A
# R: setting up B
# R: setting up C
# R: foka

Another one related to this bug: https://cfengine.com/bugtracker/view.php?id=904

You need to condition getindices() so that it happens on the second pass, like this:

body common control {
        bundlesequence => {
                            base,
                            g,
                          };

        inputs => {
                    "/var/cfengine/inputs/cfengine_stdlib.cf",
                  };
                    }


bundle agent base {
  vars:
      "sys" slist => {
                       "A", "B", "C"
                     };

  commands:
      "/usr/bin/true"
        classes => if_ok("${sys}");

  reports:
    cfengine::
      "setting up ${sys}";
                  }

bundle agent g {
  vars:
    A::
      "fok[1]" string => "foka";


    B::
      "fok[2]" string => "fokb";


    C::
      "fok[3]" string => "fokc";

    secondpass::
      "fok_idx" slist => getindices("fok");
  classes:
      "secondpass" expression => "any";
  reports:
    cfengine::
      "foks: ${fok_idx}";
      "${fok[1]}";
               }