Allow override of defaults for slice arguments

This commit fixes a bug where if a multiple value argument (slice) has default
values, the submitted values will be appended to the default. Not
overriding them as expected.

parse.go

@@ -322,6 +322,11 @@ func setSlice(dest reflect.Value, values []string) error {
 		elem = elem.Elem()
 	}
 
+	// Truncate the dest slice in case default values exist
+	if !dest.IsNil() {
+		dest.SetLen(0)
+	}
+
 	for _, s := range values {
 		v := reflect.New(elem)
 		if err := setScalar(v.Elem(), s); err != nil {

parse_test.go

@@ -250,6 +250,19 @@ func TestMultipleWithEq(t *testing.T) {
 	assert.Equal(t, []string{"x"}, args.Bar)
 }
 
+func TestMultipleWithDefault(t *testing.T) {
+	var args struct {
+		Foo []int
+		Bar []string
+	}
+	args.Foo = []int{42}
+	args.Bar = []string{"foo"}
+	err := parse("--foo 1 2 3 --bar x y z", &args)
+	require.NoError(t, err)
+	assert.Equal(t, []int{1, 2, 3}, args.Foo)
+	assert.Equal(t, []string{"x", "y", "z"}, args.Bar)
+}
+
 func TestExemptField(t *testing.T) {
 	var args struct {
 		Foo string