encoding/json: The result has been changed even an error happened at json.Unmarshal

[brotherbin]

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

go version go1.12.6 darwin/amd64

Does this issue reproduce with the latest release?

Yes, I had try it at go version go1.13 linux/amd64

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

go env Output

GOARCH="amd64"
GOBIN="/Users/dobbin/go/bin"
GOCACHE="/Users/dobbin/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/dobbin/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/dobbin/watermirror/code/pdes-mas/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-L/Users/dobbin/Library/mpich-3.2.1 -lmpich"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/05/5bjcpjgx39j2029gw9q2xq1w0000gn/T/go-build041111828=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

type A struct {
	Id int `json:"id"`
}

func main() {
	var arr []A
	var str = `[1,2,3]`
	if err := json.Unmarshal([]byte(str), &arr); err != nil {
		fmt.Println(err)
	}
	fmt.Println(arr)
}

https://play.golang.org/p/QEWC05EBLBs

What did you expect to see?

[]

The variable arr should be a empty slice because an error happened at json.Unmarshal([]byte(str), &arr).

What did you see instead?

[{0} {0} {0}]

The variable arr is a non-empty slice with 3 elements

[bcmills]

This appears to be working as documented.

Per https://golang.org/pkg/encoding/json/#Unmarshal:

If a JSON value is not appropriate for a given target type, or if a JSON number overflows the target type, Unmarshal skips that field and completes the unmarshaling as best it can. If no more serious errors are encountered, Unmarshal returns an UnmarshalTypeError describing the earliest such error.