new check: Err from initialization statement not used

[jsageryd]

Instead of this:

if err := bar(); err != nil {
	log.Fatal(err)
}

I recently did this, using a previously declared err by mistake:

if bar(); err != nil {
	log.Fatal(err)
}

Is this something that staticcheck would be able to detect?

errcheck already in a way catches this case so arguably that could be enough, but errcheck (1) tends to report many "false positives" for errors that we might be ignoring on purpose, and (2) afaik only checks that the return value is assigned to something without caring about the context, missing something like (and this is a contrived example, but to illustrate):

if err2 := bar(); err != nil {
	log.Fatal(err2)
}

Full example: https://go.dev/play/p/0mhNf-1UKEw

[dominikh]

At a minimum this would be a good fit for #1102. It might even make sense as a stricter check (that is, for the SA category), at least when we can prove that err hasn't escaped and that bar can't possibly modify its value.