Comment 1:

The real bug is that json is using reflect incorrectly.
This is a bug in json itself.  What type are you passing to Unmarshal?

Comment 2:

I am very sorry: I have lost the code that reproduces this problem. I have just spent
~10 minutes to recall it, but I can't.

Comment 3:

My guess is that you passed a struct (not a struct pointer) to
encoding/json and that the string was one of the fields in the struct.
It would be good to test this theory and if so make Unmarshal give a
better error.
I will do this if no one else beats me to it, but probably not soon.

Labels changed: added go1.0.2.

Comment 4:

You're right. I have attached the reproducer. It currently fails with the same stack
trace:
panic: reflect.Value.SetString using unaddressable value [recovered]
    panic: interface conversion: string is not error: missing method Error
goroutine 1 [running]:
encoding/json._func_001(0x7fa2a20e0e88, 0x7fa2a1fc0100, 0x7fa2a1fc0fb8, 0x7fa2a1fc0b98)
    /usr/local/go/src/pkg/encoding/json/decode.go:123 +0xaf
----- stack segment boundary -----
reflect.flag.mustBeAssignable(0x182, 0x1800000000)
    /usr/local/go/src/pkg/reflect/value.go:268 +0x166
reflect.Value.SetString(0x4626a0, 0xf84002e230, 0x182, 0xf84003c130, 0x200000002, ...)
    /usr/local/go/src/pkg/reflect/value.go:1336 +0x25
encoding/json.(*decodeState).literalStore(0xf840059000, 0xf84002e24a, 0x600000004,
0x4626a0, 0xf84002e230, ...)
    /usr/local/go/src/pkg/encoding/json/decode.go:651 +0x730
----- stack segment boundary -----
encoding/json.(*decodeState).literal(0xf840059000, 0x4626a0, 0xf84002e230, 0x182)
    /usr/local/go/src/pkg/encoding/json/decode.go:581 +0xb7
encoding/json.(*decodeState).value(0xf840059000, 0x4626a0, 0xf84002e230, 0x182)
    /usr/local/go/src/pkg/encoding/json/decode.go:252 +0x1b1
encoding/json.(*decodeState).object(0xf840059000, 0x473fd8, 0xf84002e230, 0x192)
    /usr/local/go/src/pkg/encoding/json/decode.go:550 +0xa1c
encoding/json.(*decodeState).value(0xf840059000, 0x461198, 0xf84002e220, 0x146)
    /usr/local/go/src/pkg/encoding/json/decode.go:249 +0x1da
encoding/json.(*decodeState).object(0xf840059000, 0x473f60, 0xf84002e220, 0x196)
    /usr/local/go/src/pkg/encoding/json/decode.go:550 +0xa1c
encoding/json.(*decodeState).value(0xf840059000, 0x4600b8, 0xf84002e220, 0x160)
    /usr/local/go/src/pkg/encoding/json/decode.go:249 +0x1da
encoding/json.(*decodeState).unmarshal(0xf840059000, 0x4600a8, 0xf84002e220, 0x0, 0x0,
...)
    /usr/local/go/src/pkg/encoding/json/decode.go:136 +0x159
encoding/json.Unmarshal(0xf84002e240, 0x1000000010, 0x4600a8, 0xf84002e220,
0xf84002e230, ...)
    /usr/local/go/src/pkg/encoding/json/decode.go:65 +0xef
main.main()
    /home/krasin/json.go:11 +0x11c
goroutine 2 [syscall]:
created by runtime.main
    /usr/local/go/src/pkg/runtime/proc.c:221
exit status 2

Attachments:

1. json.go (291 bytes)

Comment 5:

Owner changed to builder@golang.org.

Status changed to Accepted.

Comment 6:

Here's a smaller test case:
func TestInterfaceSet(t *testing.T) {
    b := struct{ X interface{} }{"foo"}
    const blob = `{"X":"bar"}`
    if err := Unmarshal([]byte(blob), &b); err != nil {
        t.Errorf("%v", err)
    }
}
It's trying to update a string value that isn't addressable because it's stored as an
interface value. The containing struct is addressable, so the json package could be
clever enough to walk back up and the value correctly. Not sure that it's worth it,
though.

Comment 7:

Thank you for the smaller case. Nice.

Comment 8:

This issue was closed by revision 09b736a.

Status changed to Fixed.