encoding/json: wrong encoding for json.Number field with string option (struct tag)

[breml]

If the string option is set on a struct field of type json.Number, the value gets encoded without quotes. If this encoded value is then unmarshaled back, it fails, because the expected quotes are missing.

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

$ go version
go version go1.12.5 linux/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="/home/lubr/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/lubr/go"
GOPROXY=""
GORACE=""
GOROOT="/home/lubr/.gvm/versions/go1.12.5.linux.amd64"
GOTMPDIR=""
GOTOOLDIR="/home/lubr/.gvm/versions/go1.12.5.linux.amd64/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build368255501=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package main

import (
	"encoding/json"
	"fmt"
	"log"
)

type stringTest struct {
	JSONNumberStringOption json.Number `json:",string"`
}

func main() {
	b, err := json.Marshal(stringTest{
		JSONNumberStringOption: "2",
	})
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(string(b))

	var res stringTest
	err = json.Unmarshal(b, &res)
	if err != nil {
		log.Fatal(err)
	}
}

https://play.golang.org/p/iX9v1XWpsLG

What did you expect to see?

{"JSONNumberStringOption":"2"}

What did you see instead?

{"JSONNumberStringOption":2}
2019/09/12 21:33:02 json: invalid use of ,string struct tag, trying to unmarshal unquoted value into json.Number
exit status 1

[gopherbot]

Change https://golang.org/cl/195043 mentions this issue: encoding/json: fix marshal for json.Number ,string

[mvdan]

The fact that you can encode but then not decode back seems to point clearly at a bug here; thanks for spotting this.

[mdempsky]

I added and then quickly deleted this comment, but I think a few people still saw it via email notifications, so I figured I might as well restore it for posterity:

Package json's documentation describes:

The "string" option signals that a field is stored as JSON inside a JSON-encoded string. It applies only to fields of string, floating point, integer, or boolean types.

So using it for json.Number is technically against the documentation.

I don't have any preference here; but if json.Number is updated to support the "string" option, then this documentation should also be updated.

I made this comment before realizing that Number is a declared type whose underlying type is string. After realizing that, however, I didn't feel confident about my interpretation and wanted to retract it, rather than add uninformed drive-by chatter. (But you can't delete anything from the Internet.)