Cannot convert []string to []interface {}

This is not a bug. fmt.Println() requires a []interface{} type. That means, it must be a slice of interface{} values and not “any slice”. In order to convert the slice, you will need to loop over and copy each element.

old := flag.Args()
new := make([]interface{}, len(old))
for i, v := range old {
    new[i] = v
}
fmt.Println(new...)

The reason you can’t use any slice is that conversion between a []string and a []interface{} requires the memory layout to be changed and happens in O(n) time. Converting a type to an interface{} requires O(1) time. If they made this for loop unnecessary, the compiler would still need to insert it.