encoding/json: panic: reflect.Set: value of type []uint8 is not assignable to type []main.Uint8

[kshvakov]

go version go1.5 darwin/amd64
go version go1.5.1 linux/amd64

package main

import (
    "encoding/json"
)

type Uint8 uint8

func main() {

    var sliceIn, sliceOut []Uint8

    for i := 0; i < 20; i++ {

        sliceIn = append(sliceIn, Uint8(i))
    }

    j, _ := json.Marshal(sliceIn)

    json.Unmarshal(j, &sliceOut)
}

output

panic: reflect.Set: value of type []uint8 is not assignable to type []main.Uint8 [recovered]
    panic: interface conversion: string is not error: missing method Error

goroutine 1 [running]:
encoding/json.(*decodeState).unmarshal.func1(0x820275e40)
    /usr/local/go/src/encoding/json/decode.go:145 +0xba
reflect.Value.assignTo(0xdf2c0, 0x8202ae0c0, 0x57, 0x13d0e0, 0xb, 0xde2a0, 0x0, 0x0, 0x0, 0x0)
    /usr/local/go/src/reflect/value.go:2158 +0x3be
reflect.Value.Set(0xde2a0, 0x8202ae000, 0xd7, 0xdf2c0, 0x8202ae0c0, 0x57)
    /usr/local/go/src/reflect/value.go:1327 +0x95
encoding/json.(*decodeState).literalStore(0x8202ba000, 0x8202b0024, 0x1e, 0x40, 0xde2a0, 0x8202ae000, 0xd7, 0x0)
    /usr/local/go/src/encoding/json/decode.go:760 +0x14ee
encoding/json.(*decodeState).literal(0x8202ba000, 0xd81e0, 0x8202ae000, 0x16)
    /usr/local/go/src/encoding/json/decode.go:646 +0xe9
encoding/json.(*decodeState).value(0x8202ba000, 0xd81e0, 0x8202ae000, 0x16)
    /usr/local/go/src/encoding/json/decode.go:299 +0x397
encoding/json.(*decodeState).unmarshal(0x8202ba000, 0xd81e0, 0x8202ae000, 0x0, 0x0)
    /usr/local/go/src/encoding/json/decode.go:157 +0x196
encoding/json.Unmarshal(0x8202b0024, 0x1e, 0x40, 0xd81e0, 0x8202ae000, 0x0, 0x0)
    /usr/local/go/src/encoding/json/decode.go:85 +0x12b
main.main()
    /Users/kshvakov/unmarshal_bug.go:20 +0x168
exit status 2

http://play.golang.org/p/ZFlbNkI0H3

[freeformz]

FWIW: This worked on go 1.4.2

[adg]

Confirmed that it works under Go 1.4, so this is a regression.

Not sure what to do about this, though, because you according to the language spec you can't assign []T to []S, even if T and S have the same underlying type. The reflect package is supposed to enforce the language spec.

cc @rsc @ianlancetaylor @griesemer

[kshvakov]

@adg this problem only occurs with type uint8

https://golang.org/src/encoding/json/decode.go?s=#L749

example

package main

import (
    "encoding/json"
    "fmt"
)

type MyType int8

func main() {

    var sliceIn, sliceOut []MyType

    for i := 0; i < 20; i++ {

        sliceIn = append(sliceIn, MyType(i))
    }

    j, _ := json.Marshal(sliceIn)

    json.Unmarshal(j, &sliceOut)

    fmt.Printf("%#v\n", sliceOut)
}

output

[]main.MyType{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19}

http://play.golang.org/p/ZTIYhhnbVI

[adg]

Well, that's weird.

[kshvakov]

@adg

package main

import (
    "encoding/json"
    "fmt"
)

type Uint8 uint8

func main() {

    var sliceIn []Uint8

    for i := 0; i < 20; i++ {

        sliceIn = append(sliceIn, Uint8(i))
    }

    j, _ := json.Marshal(sliceIn)

    fmt.Println(string(j))
}

output

"AAECAwQFBgcICQoLDA0ODxAREhM="

[adg]

I guess it's related to encoding/json's special handling of []byte.

On 14 October 2015 at 09:04, Kirill Shvakov notifications@github.com
wrote:

@adg https://github.com/adg

package main
import (
"encoding/json"
"fmt"
)
type Uint8 uint8
func main() {

var sliceIn []Uint8

for i := 0; i < 20; i++ {

    sliceIn = append(sliceIn, Uint8(i))
}

j, _ := json.Marshal(sliceIn)

fmt.Println(string(j))

}

output

"AAECAwQFBgcICQoLDA0ODxAREhM="

—
Reply to this email directly or view it on GitHub
#12921 (comment).

[kshvakov]

it seems that 4302fd0 ?

[adg]

Yeah, it's the fix to #8962 but it's debatable as to whether we fixed it in the right direction.

[gopherbot]

CL https://golang.org/cl/16303 mentions this issue.

[dsturrock]

I don't think it was introduced in encoding/json, just tried adding the same type declaration and managed to replicate it myself:

panic: reflect.Set: value of type []uint8 is not assignable to type []main.Uint8 [recovered]
    panic: reflect.Set: value of type []uint8 is not assignable to type []main.Uint8

goroutine 5 [running]:
testing.tRunner.func1(0xc8200921b0)
    /usr/local/go/src/testing/testing.go:450 +0x171
reflect.Value.assignTo(0x6c88c0, 0xc82000ebe0, 0x57, 0x7e8f90, 0xb, 0x6c5f80, 0x0, 0x0, 0x0, 0x0)
    /usr/local/go/src/reflect/value.go:2158 +0x3be
reflect.Value.Set(0x6c5f80, 0xc82000eba0, 0xd7, 0x6c88c0, 0xc82000ebe0, 0x57)
    /usr/local/go/src/reflect/value.go:1327 +0x95
database/sql.convertAssign(0x6b6c20, 0xc82000eba0, 0x6c88c0, 0xc82000ebe0, 0x0, 0x0)
    /usr/local/go/src/database/sql/convert.go:207 +0x7ca
database/sql.(*Rows).Scan(0xc82001e540, 0xc82000b340, 0x1, 0x1, 0x0, 0x0)
    /usr/local/go/src/database/sql/sql.go:1692 +0x47a