cmd/vet: incorrect printf "invalid argument index" with ... argument

What version of Go are you using (go version)?

tip (79669dc705)

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

darwin/amd64

What did you do?

https://play.golang.org/p/uIeIGQWBhgB

package main

import (
	"fmt"
)

func main() {
	s := []interface{}{"Hello", "playground"}
	fmt.Printf("%[2]s\n", s...)
}

What did you expect to see?

No vet warning.

I think this is a valid program, which prints "playground" as intended. It doesn't seem like a potential bug to me.

What did you see instead?

./prog.go:9:2: Printf format has invalid argument index [2]