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.

In this case, a type conversion is unnecessary. Simply pass the flag.Args() value to fmt.Println.

---

Question:

Cannot convert []string to []interface {}

I'm writing some code, and I need it to catch the arguments and pass them through fmt.Println (I want its default behaviour, to write arguments separated by spaces and followed by a newline).

Here's the code example:

package main

import (
    "fmt"
    "flag"
)

func main() {
    flag.Parse()
    fmt.Println(flag.Args()...)
}

---

Package flag

import "flag"

func Args

func Args() []string

Args returns the non-flag command-line arguments.

---

Package fmt

import "fmt"

func Println

func Println(a ...interface{}) (n int, err error)

Println formats using the default formats for its operands and writes to standard output. Spaces are always added between operands and a newline is appended. It returns the number of bytes written and any write error encountered.

---

In this case, a type conversion is unnecessary. Simply pass the flag.Args() value to fmt.Println, which uses reflection to interpret the value as type []string. Package reflect implements run-time reflection, allowing a program to manipulate objects with arbitrary types. For example,

args.go:

package main

import (
    "flag"
    "fmt"
)

func main() {
    flag.Parse()
    fmt.Println(flag.Args())
}

Output:

$ go build args.go
$ ./args arg0 arg1
[arg0 arg1]
$ 

If it's only a slice of strings you want to print, you can avoid conversion and get the exact same output by joining:

package main

import (
    "fmt"
    "flag"
    "strings"
)

func main() {
    flag.Parse()
    s := strings.Join(flag.Args(), " ")
    fmt.Println(s)
}