ber.go
func setDefaultValue(v reflect.Value, params fieldParameters) (ok bool) {
	if !params.optional {
		return
	}
	ok = true
	if params.defaultValue == nil {
		return
	}
	if canHaveDefaultValue(v.Kind()) {
		v.SetInt(*params.defaultValue)
	}
	return
}
func setDefaultValue(v reflect.Value, params fieldParameters) (ok bool) {
	if !params.optional {
		return
	}
	ok = true
	if params.defaultValue == nil {
		return
	}
	if canHaveDefaultValue(v.Kind()) {
		v.SetInt(*params.defaultValue)
	}
	return
}
// setDefaultValue is used to install a default value, from a tag string, into
// a Value. It is successful if the field was optional, even if a default value
// wasn't provided or it failed to install it into the Value.