encoding/json: unexpected behavior when unmarshalling to "boxed" struct

[brenol]

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

$ go version
go version go1.12.9 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/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/goworkspace"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/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-build909978713=/tmp/go-build -gno-record-gcc-switches"

What did you do?

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

What did you expect to see?

I did not expect v to be of type map[string]interface{}, as its underlying type is clearly Y.

What did you see instead?

vis of type map[string]interface{}, failing the type assertion.

Note that if it's a pointer to Y, it works correctly.
Also, this came from a bigger project where Sprintf was reporting both vand Y{} with the same underlying type, however, I was unable to reproduce that here.

Also, removing json.Unmarshal fixes the issue, as the boxed Y is cast correctly to Y when there is no unmarshal.

[OneOfOne]

You need to use a pointer to Y:

https://play.golang.org/p/wYxDOs4QdUn

[katiehockman]

@brenol can you confirm that this suggestion resolves your issue? If so, this issue can be closed.

You can also check out https://github.com/golang/go/wiki/Questions for more resources.

[brenol]

Yes, the suggestion fixes the issue.

However, is this the expected behavior? I mean, as I mentioned, removing json.Unmarshal altogether does work and it does not panic.

My main issue here is: should json.Unmarshal change the underlying type of a by value struct to map[string]interface{}? I tried looking up for some reason this happens but it didn't make sense to me, as the docs don't make it clear why it works with pointers and not with values.

[katiehockman]

/cc @rsc

[seankhliao]

Plain structs in interfaces aren't addressable, so they can't be set.
encoding/json makes no provision for setting the value of non pointer structs (since it can't), so it follows the other documented rules, allocating a map.