After throws unhelpful error message on passing invalid index

[BaibhaVatsa]

Issue description

On passing negative indices, throws error message which does not accurately explain the error. Something like "cannot return slice of elements after a negative index" would be more explanatory in my opinion. Any suggestions?

If this gets the approval, I will be more than happy to submit a PR.

What version of Hugo are you using (hugo version)?

$ hugo version
Hugo Static Site Generator v0.59.0-DEV-096A4B67 linux/amd64 BuildDate: 2019-10-11T02:36:33-0500

Does this issue reproduce with the latest release?

Yes

[bep]

I agree that the error isn't great. Another problem with these related "slice functions" (After, First and probably others) is that they return slightly different error messages for the same issue.

Maybe we can get some input from the Go compiler and start clean this up.

https://play.golang.org/p/P8aYI5j7lxp

package main

import (
	"fmt"
)

func main() {
	slice(-1, 2)
	slice(0, 20)
}

var s = []string{"a", "b", "c"}

func slice(a, b int) {
	defer func() {
		if r := recover(); r != nil {
			fmt.Printf("%d:%d => %s\n", a, b, r)
		}
	}()
	fmt.Println(s[a:b])
}

Which prints:

-1:2 => runtime error: slice bounds out of range [-1:]
0:20 => runtime error: slice bounds out of range [:20] with capacity 3

So for your After example, maybe:

sequence bounds out of range [-1:]

[BaibhaVatsa]

I feel that the suggested error message makes more sense for after. Should I submit a PR with the change for after?

First throws which I feel can be changed to something like

sequence length must be non-negative

to resemble more of what the Go compiler would throw.

[bep]

Sounds good.

[BaibhaVatsa]

I was thinking that once #6419 is closed, I will change also to

sequence length must be non-negative

as it would have the same condition as first then.

[bep]

I agree.

[github-actions]

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