proposal: encoding/json: add error var to compare the returned error when using json.Decoder.DisallowUnknownFields()

[jaswdr]

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

$ go version

go version devel +ffc7bc55f3 Tue Oct 9 10:35:08 2018 +0000 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/jaswdr/.cache/go-build"

GOEXE=""

GOFLAGS=""

GOHOSTARCH="amd64"

GOHOSTOS="linux"

GOOS="linux"

GOPATH="/home/jaswdr"

GOPROXY=""

GORACE=""

GOROOT="/home/jaswdr/go"

GOTMPDIR=""

GOTOOLDIR="/home/jaswdr/go/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-build749620831=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package main

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

type T struct {
	Bar string `json:"bar"`
}

func main() {
	str := []byte(`{"foo":"bar"}`)
	obj := new(T)
	
	dec := json.NewDecoder(bytes.NewReader(str))
	dec.DisallowUnknownFields()
	err := dec.Decode(&obj)
	if err != nil { // Whant to check if the specific unkown fields error happen
		log.Fatal(err)
	}
	
	log.Println(obj)
}

What did you expect to see?

Some way to check if the error happen.

...
if err == json.UnknownFieldsError {
 ...
}
...

What did you see instead?

No way to do this instead of checking the error string

---

If this is accepted I really want to work on it.

[gopherbot]

Change https://golang.org/cl/152297 mentions this issue: encoding/json: created json.UnknownFieldsError

[andybons]

Per discussion with @golang/proposal-review, putting on hold until we can review JSON issues en masse.