Remove redundant ExitOnError output (#2)

flag.go

@@ -1199,7 +1199,6 @@ func (f *FlagSet) Parse(arguments []string) error {
 		case ContinueOnError:
 			return err
 		case ExitOnError:
-			fmt.Println(err)
 			os.Exit(2)
 		case PanicOnError:
 			panic(err)

flag_test.go

@@ -7,6 +7,7 @@ import (
 	"io/ioutil"
 	"net"
 	"os"
+	"os/exec"
 	"reflect"
 	"sort"
 	"strconv"
@@ -897,6 +898,34 @@ func TestOutput(t *testing.T) {
 	}
 }
 
+func TestOutputExitOnError(t *testing.T) {
+	if os.Getenv("PFLAG_CRASH_TEST") == "1" {
+		CommandLine = NewFlagSet(t.Name(), ExitOnError)
+		os.Args = []string{t.Name(), "--unknown"}
+		Parse()
+		t.Fatal("this error should not be triggered")
+		return
+	}
+	mockStdout := bytes.NewBufferString("")
+	mockStderr := bytes.NewBufferString("")
+	cmd := exec.Command(os.Args[0], "-test.run="+t.Name())
+	cmd.Env = append(os.Environ(), "PFLAG_CRASH_TEST=1")
+	cmd.Stdout = mockStdout
+	cmd.Stderr = mockStderr
+	err := cmd.Run()
+	if e, ok := err.(*exec.ExitError); ok && !e.Success() {
+		want := "unknown flag: --unknown\nUsage of " + t.Name() + ":\n"
+		if got := mockStderr.String(); got != want {
+			t.Errorf("got '%s', want '%s'", got, want)
+		}
+		if got := mockStdout.String(); len(got) != 0 {
+			t.Errorf("stdout should be empty, got: %s", got)
+		}
+		return
+	}
+	t.Fatal("this error should not be triggered")
+}
+
 // This tests that one can reset the flags. This still works but not well, and is
 // superseded by FlagSet.
 func TestChangingArgs(t *testing.T) {