cmd/vet: stdmethods check gets confused if run on a package named "xml"

[jacoelho]

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

$ go version
go version go1.11.4 darwin/amd64

Does this issue reproduce with the latest release?

yes

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

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/jc/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/jc/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.11.4/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.11.4/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/pf/_mdvc42j193c4tf6kgfm80gcss_jqd/T/go-build565539251=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

For example, if I have the following package:

package xml

import (
	"encoding/xml"
	"io"
)

type XMLMap map[string]string

type XMLMapEntry struct {
	XMLName xml.Name
	Value   string `xml:",chardata"`
}

// UnmarshalXML 
func (m *XMLMap) UnmarshalXML(dec *xml.Decoder, start xml.StartElement) error {
	v := XMLMap{}

	for {
		var e XMLMapEntry

		err := dec.Decode(&e)
		if err == io.EOF {
			break
		}

		if err != nil {
			return err
		}

		v[e.XMLName.Local] = e.Value
	}

	*m = v
	return nil
}

What did you expect to see?

go vet ./...
< no output>

What did you see instead?

$ go vet ./...
./xml.go:16: method UnmarshalXML(dec *xml.Decoder, start xml.StartElement) error should have signature UnmarshalXML(*xml.Decoder, xml.StartElement) error

even if I try to alias encoding/xml import I still get same error.

If instead I have the same code, but rename package:

package something
...
<same code as before>

go vet ./...
< no output>

go vet doesn't seem to handle correctly if code is in package also named xml.

[mvdan]

Interesting. Like you say, this sounds like a bug in vet relating to how it identifies or compares types.

[jpopadak]

I have a very similar issue. I have a package named github.com/mywork/myservice/encoding/json and I have my own Unmarshaler interface.

// Marshaler allows a type to force how it is encoded, given it knows how do the JSON Schema encoding and return
// its schemaID.
type Marshaler interface {
	// MarshalJSON knows the schemaID and how to encode itself in JSON format.
	MarshalJSON() (encodedBytes []byte, schemaID int, err error)
}

I get the following.

go vet ./...
# github.com/mywork/myservice/encoding/json
encoding/json/encode.go:17:2: method MarshalJSON() (encodedBytes []byte, schemaID int, err error) should have signature MarshalJSON() ([]byte, error)

For my code, I require a schemaID to be returned. We have many different types (Avro, JSON, plaintext, Protobuf) and each of them have their own Marshaler. They all exist within our project github.com/mywork/myservice/encoding/* package. The error is not thrown for the other types, assuming because they have (not std lib) MarshalAvro and MarshalProto methods.

Is there any workaround for this? The only one I currently see is to disable the -stdmethods on go vet.