Fix defaultIsZeroValue check for generic Value types

flag.go

@@ -551,7 +551,7 @@ func (f *Flag) defaultIsZeroValue() bool {
 	case *intSliceValue, *stringSliceValue, *stringArrayValue:
 		return f.DefValue == "[]"
 	default:
-		switch f.Value.String() {
+		switch f.DefValue {
 		case "false":
 			return true
 		case "<nil>":

flag_test.go

@@ -1183,6 +1183,7 @@ const defaultOutput = `      --A                         for bootstrapping, allo
       --StringSlice strings       string slice with zero default
       --Z int                     an int that defaults to zero
       --custom custom             custom Value implementation
+      --custom-with-val custom    custom value which has been set from command line while help is shown
       --customP custom            a VarP with default (default 10)
       --maxT timeout              set timeout for dial
   -v, --verbose count             verbosity
@@ -1234,12 +1235,18 @@ func TestPrintDefaults(t *testing.T) {
 	cv2 := customValue(10)
 	fs.VarP(&cv2, "customP", "", "a VarP with default")
 
+	// Simulate case where a value has been provided and the help screen is shown
+	var cv3 customValue
+	fs.Var(&cv3, "custom-with-val", "custom value which has been set from command line while help is shown")
+	err := fs.Parse([]string{"--custom-with-val", "3"})
+	if err != nil {
+		t.Error("Parsing flags failed:", err)
+	}
+
 	fs.PrintDefaults()
 	got := buf.String()
 	if got != defaultOutput {
-		fmt.Println("\n" + got)
-		fmt.Printf("\n" + defaultOutput)
-		t.Errorf("got %q want %q\n", got, defaultOutput)
+		t.Errorf("\n--- Got:\n%s--- Wanted:\n%s\n", got, defaultOutput)
 	}
 }