Combining explicit arguments with slice expansion doesn't work

[funny-falcon]

Probably it is already discussed. But I didn't see description in Language Specification.

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

go version go1.8 linux/amd64
play.golang.org

Does this issue reproduce with the latest release?

I think, yes.

What did you do?

https://play.golang.org/p/2Qm34TE-gn

func main() {
	a := []interface{}{ 1, 2, 3 }
	fmt.Println(1, 2, 3)
	fmt.Println(a...)
	fmt.Println(-1, 0, a...)
}

What did you expect to see?

1 2 3
1 2 3
-1 0 1 2 3

What did you see instead?

tmp/sandbox223672132/main.go:11:13: too many arguments in call to fmt.Println
	have (number, number, []interface {}...)
	want (...interface {})

[robpike]

This is working as defined in the spec. The arguments to match a variadic argument may be a list of singletons or a slice followed by ..., not a combination. The main reason is to avoid ambiguity that can lead to errors.

Working as intended.

[mvdan]

The previous Go2 proposals are #18605 and #19218.

@robpike you might want to raise your point in the proposals, or when the time comes for Go2.