fix #1121(StringSliceFlag set default value into destination)

flag_string_slice.go

@@ -116,6 +116,14 @@ func (f *StringSliceFlag) GetValue() string {
 
 // Apply populates the flag given the flag set and environment
 func (f *StringSliceFlag) Apply(set *flag.FlagSet) error {
+
+	if f.Destination != nil {
+		if f.Value != nil {
+			f.Destination.slice = make([]string, len(f.Value.slice))
+			copy(f.Destination.slice, f.Value.slice)
+		}
+	}
+
 	if val, ok := flagFromEnvOrFile(f.EnvVars, f.FilePath); ok {
 		f.Value = &StringSlice{}
 		destination := f.Value

flag_test.go

@@ -386,6 +386,18 @@ func TestStringSliceFlagApply_SetsAllNames(t *testing.T) {
 	expect(t, err, nil)
 }
 
+func TestStringSliceFlagApply_DefaultValueWithDestination(t *testing.T) {
+	defValue := []string{"UA", "US"}
+
+	fl := StringSliceFlag{Name: "country", Value: NewStringSlice(defValue...), Destination: NewStringSlice("CA")}
+	set := flag.NewFlagSet("test", 0)
+	_ = fl.Apply(set)
+
+	err := set.Parse([]string{})
+	expect(t, err, nil)
+	expect(t, defValue, fl.Destination.Value())
+}
+
 var intFlagTests = []struct {
 	name     string
 	expected string