encoding/json: Decoding returns error when case-insensitive keys overlap and not all struct fields defined

[sdcoffey]

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

1.18

Does this issue reproduce with the latest release?

Yes, reproducible with go 1.19

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

go env Output

GO111MODULE="on"
GOARCH="arm64"
GOBIN=""
GOCACHE="/Users/steve/Library/Caches/go-build"
GOENV="/Users/steve/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/steve/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/steve/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.18"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/steve/yuzu/crypto-stream-source/go.mod"
GOWORK=""
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 arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/q_/381gr8255wl427x33rzsyvk80000gn/T/go-build2527350202=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Go Playground

Given this block of JSON (example taken from a Binance websocket payload):

{
  "e": "24hrTicker", 
  "E": 123456789
}

encoding/json will return an error while unmarshaling into this struct:

type UnsuccessfulUnmarshal struct {
	TimeStamp int64 `json:"E"`
}

but not this struct:

type SuccessfulUnmarshal struct {
	Event     string `json:"e"`
	TimeStamp int64  `json:"E"`
}

What did you expect to see?

No error returned when only some of the fields in the json payload are defined in the destination struct

What did you see instead?

This error returned:

json: cannot unmarshal string into Go struct field UnsuccessfulUnmarshal.E of type int64

[seankhliao]

Duplicate of #14750