cmd/vet: shadow false positive variable/type declaration in package block

[meomap]

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

go version go1.7.3 darwin/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/meomap/testlinter"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/6f/tq_z_9xs71vg9wlv5yg8g_lm0000gn/T/go-build230941075=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

What did you do?

I run go tool vet --shadow on top repository of 3 packages, each has a main.go file:

// `a/main.go`
package main

var foo error

func main() {
}

// `b/main.go`
package main

import (
        "errors"
        "fmt"
)

func main() {
        foo := errors.New("meomeo")
        fmt.Println(foo)
}

// `c/main.go`
package main

import "errors"

type foo error

func newFoo(msg string) foo {
        return foo(errors.New(msg))
}

func main() {
}

vet command

$ go tool vet --shadow a/main.go b/main.go c/main.go 

What did you expect to see?

no vet error, same output as

$ go tool vet --shadow .

What did you see instead?

b/main.go:9: declaration of "foo" shadows declaration at a/main.go:3

but when I run same vet on a/main.go and b/main.go only, no vet error

$ go tool vet --shadow a/main.go b/main.go 

or change order of arguments input, no vet error

$ go tool vet --shadow c/main.go b/main.go a/main.go

[mvdan]

Note that go help packages says:

As a special case, if the package list is a list of .go files from a
single directory, the command is applied to a single synthesized
package made up of exactly those files, ignoring any build constraints
in those files and ignoring any other files in the directory.

This behaviour is still confusing and perhaps a bug, but I would also say that you are not using the tool properly by specifying go files from different packages. That merges them into a single package. Have you tried ./...?

[mvdan]

I just realised that it says from a single directory. I wonder if that means that it treats all the files as if they were from the same directory, or if it does a separate run for each different directory. I'm betting on the former.

[robpike]

There is no expectation that any of the go tools will work properly when presented with a single directory containing multiple packages.

[meomap]

@mvdan actually go vet can run with . only. Anyway thanks for sharing that package usage note

[mvdan]

@meomap like any other go foo tool, go vet ./... works just fine here. Make sure your GOPATH is properly set up and you're in it.