cmd/vet: warning with duplicate `json:"omitempty"` field tag

[ls42]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.9.2 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/stephan/workspace/go"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.9.2/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.9.2/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/k7/b4_kf50n6p9cjdb_6r1sr91c0000gn/T/go-build245233983=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

Write the following code in VSCode:

// Host represents a computer, a VM or a similar system
type Host struct {
	Name          string `json:"hostname"`
	Domain        string `json:"domain"`
	Login         string `json:"login"`
	Contact       string `json:"contact"`
	CPUCores      uint   `json:"sp_number_processors"`
	CPUSpeed      string `json:"sp_current_processor_speed"`
	Memory        string `json:"memorysize_mb"`
	FacterVersion string `json:"facterversion"`
	Kernel        string `json:"kernelrelease"`
	Release       string `json:"operatingsystem"`
	Serial        string `json:"serialnumber"`
	IsVirtual     bool   `json:"is_virtual"`
	Disks         []Disk `json:"omitempty"`
	ChildVMs      []Host `json:"omitempty"`
}

Then save, which will trigger go tool vet on the file. The last line (ChildVMs...) will then be highlighted with a curly green line, indicating a warning, saying it has a duplicate tag that's already defined.

What did you expect to see?

No curly green line on the last field definition since it should be OK to have multiple lines with that tag.

Over at the repository for the Go-VSCode plugin I posted a screenshot.

[ALTree]

You are missing the leading comma

json:",omitempty"

I believe it's required.

[bradfitz]

@ALTree is correct. (As is cmd/vet... it found a real problem.)