x/exp/slices: Replace doesn't panic with invalid indices

[blackgreen100]

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

Go 1.19

Does this issue reproduce with the latest release?

Yes, at tip

What did you do?

The documentation for slices.Replace states that:

[...] Replace panics if s[i:j] is not a valid slice of s.

The following program doesn't panic, instead it returns a rather unexpected output:

func main() {
	x := []int{10, 20, 30, 40}
	y := []int{3, 4}

	lo, hi := 2, 1
	// fmt.Println(x[lo:hi]) this would panic with `panic: runtime error: slice bounds out of range [2:1]`
	z := slices.Replace(x, lo, hi, y...)
	fmt.Println(z)
}

https://go.dev/play/p/hCkCLy0YZnT?v=gotip

What did you expect to see?

A panic, because the slice expression x[2:1] is out of bounds as per the specs (and as per the Replace documentation):

For arrays or strings, the indices are in range if 0 <= low <= high <= len(a), otherwise they are out of range. For slices, the upper index bound is the slice capacity cap(a) rather than the length

What did you see instead?

The output of printing z is:

[10 20 3 4 20 30 40]

(unexpected outputs occur also in the general case when j - i != len(y))

Should probably just add bounds check like slices.Delete at the beginning of the function:

_ = s[i:j] // bounds check

[mknyszek]

CC @ianlancetaylor

Despite the "Documentation" tag that gopherbot added, this just seems like a bug in that package.

[gopherbot]

Change https://go.dev/cl/449915 mentions this issue: slices: don't accept out of order indexes in Replace