encoding/json: UnmarshalTypeError Offset is broken if a nested struct implements UnmarshalJSON

[ajpetersons]

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

$ go version
go version go1.15.6 darwin/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
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/aj/Library/Caches/go-build"
GOENV="/Users/aj/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/aj/go/pkg/mod"
GONOPROXY="*private gitlab URL*"
GONOSUMDB="*private gitlab URL*"
GOOS="darwin"
GOPATH="/Users/aj/go"
GOPRIVATE="*private gitlab URL*"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/2n/c518mztn5yq_4k20jhf400vc0000gn/T/go-build993004208=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

https://play.golang.org/p/dJYX2lDwSKb

What did you expect to see?

json.UnmarshalTypeError.Offset to be the same for nested structs.

What did you see instead?

json.UnmarshalTypeError.Offset is only in the scope of nested struct if the struct implements a UnmarshalJSON method. The Offset should start from the beginning of the struct

[networkimprov]

cc @mvdan

[mvdan]

Perhaps related to #34543. I'd suggest to re-run your experiment with that fix first.

[seankhliao]

Duplicate of #11858