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

$ go version
go version go1.16.5 darwin/amd64

Does this issue reproduce with the latest release?

Yes, 1.16.5 is the latest version today!

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

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/ebi-exp/Library/Caches/go-build"
GOENV="/Users/ebi-exp/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/ebi-exp/go/1.16.5/pkg/mod"
GONOPROXY="github.com/sikiapi"
GONOSUMDB="github.com/sikiapi"
GOOS="darwin"
GOPATH="/Users/ebi-exp/go/1.16.5"
GOPRIVATE="github.com/sikiapi"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/ebi-exp/.anyenv/envs/goenv/versions/1.16.5"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/ebi-exp/.anyenv/envs/goenv/versions/1.16.5/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.16.5"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/ebi-exp/ghq/github.com/ebi-yade/go-json-sandbox/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/s7/1j_fhlc91jj470cmn36hqs980000gn/T/go-build3900691258=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Here is a pull request for my sandbox repository. As the code and test actions, you would easily find the JSON Marshal / Unmarshal functions for json.RawMessage fields behave differently from ones for []byte. And the weirdest point is parsing a field of null as the json.RawMessage type results in not nil but the byte array equivalent to a string "null". However, in the cases for other (Go-standard) types, it interprets them as the zero value of the specified type (like 0 for numbers, "" for string, and even nil for []byte).

What did you expect to see?

I want the json.RawMessage to behave as possible as similar to []byte because it's the type alias, but if this is hard, it's enough that the null value is parsed into nil of json.RawMessage, as above.

What did you see instead?

I'm looking for the implementation code of UnmarshalJSON and MarshalJSON for []byte type and want to see the difference from the ones for json.RawMessage, but for now, I couldn't find them. And at least, it's definitely clear this issue can be reproduced on Go Playground: https://play.golang.org/p/YTHmX5FOg_x