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

$ go version
go version go1.15.7 linux/amd64

Does this issue reproduce with the latest release?

Yes.

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

$ go env

What did you do?

$ cat a.go
package main

import "os"

func foo() (err error) {
        x, err := os.Getwd()
        _ = x
        return
}
$ go install golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow
$ go vet -vettool=$(which shadow) a.go
$

What did you expect to see?

Warning about err being shadowed.

What did you see instead?

Nothing.