encoding/json: Json marshaler seems to not marshal correctly if there is an embedded struct with custom marshaller

[sraghav1]

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

$ go version
go1.16.2

Does this issue reproduce with the latest release?

Yes. Checked using go1.16.5 on play.golang.org.

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/sraghav1/.cache/go-build"
GOENV="/home/sraghav1/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/sraghav1/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/sraghav1/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.16.2"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/sraghav1/src/xeon-health/go.mod"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build904965005=/tmp/go-build -gno-record-gcc-switches"

What did you do?

• Create a struct C which embeds another struct A and adds additional fields
• The struct A has a custom marshaler with pointer receiver. (func (a *A) MarshalJSON() ([]byte, error) )
• Marshalling a pointer to an object of type C, returns marshaled JSON with only A.

Example: https://play.golang.org/p/euugvbFXI76

type A struct {
	X int
	Y string
	Z float64
}

type C struct {
	X2 int
	A
}

func (a *A) MarshalJSON() ([]byte, error) {
	type aliasA A
	a2:=aliasA(*a) 
	return json.Marshal(a2) // Essentially a no-op custom marshaler to illustrate the problem
}
	
func main() {	
	a := A{X:10, Y:"faster", Z:2.43}
	c := C{X2:101, A:a}
			
	jc, _ := json.Marshal(&c)
	fmt.Println(string(jc))		
}

Additional notes

• The problem also happens if I don't use pointers (Like this: https://play.golang.org/p/kMsafmDh0ki)
• The problem does not happen if I add a custom marshaler to the type C (Like this: https://play.golang.org/p/dmst19c-N9G)

What did you expect to see?

{"X2":101,"X":10,"Y":"faster","Z":2.43}

What did you see instead?

{"X":10,"Y":"faster","Z":2.43}

[seankhliao]

The UnmarshalJSON MarshalJSON method is promoted from A to C because of embedding

Unlike many projects, the Go project does not use GitHub Issues for general discussion or asking questions. GitHub Issues are used for tracking bugs and proposals only.

For questions please refer to https://github.com/golang/go/wiki/Questions

[sraghav1]

I believe this is not a general discusison or a doubt I am trying to ask. I can take it to other forums but will be great if you can check the issue description again.

I am not talking about UnmarshalJSON but MarshalJSON.

The problem is not it is getting promoted from A to C, but it is NOT getting promoted as expected.

It gets promoted as expected when a dummy custom marshaller is added to C which is expected. But it does not get promoted if there is not custom marshaler.

[seankhliao]

Still works the same, embedded fields/methods are promoted if they don't conflict