encoding/json: possible bug when unmarshaling maps with a custom struct as their key

[skwair]

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

$ go version
go version go1.14.2 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
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/antoine/.cache/go-build"
GOENV="/home/antoine/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/antoine/go"
GOPRIVATE=""
GOPROXY=""
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
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-build123794036=/tmp/go-build -gno-record-gcc-switches"
GOROOT/bin/go version: go version go1.14.2 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.14.2
uname -sr: Linux 5.6.7-1-MANJARO
LSB Version:	n/a
Distributor ID:	ManjaroLinux
Description:	Manjaro Linux
Release:	20.0
Codename:	Lysia
/usr/lib/libc.so.6: GNU C Library (GNU libc) stable release version 2.31.

What did you do?

Here's a playground link describing the issue.

This issue seems to only occur when other.Name contains a special character (" or \ for example) after the size of the key.

What did you expect to see?

When unmarshaled, the keys are correct.

What did you see instead?

The keys are incorrect under certain circumstances.

[vaibhavsanap]

When I ran the code provided in playground link locally unmarshaling works as expected and key values printed by program are perfect

Go version which I am using for reference
go version go1.13.7 darwin/amd64
Output of code:
my-key-4
my-key-1
my-key-2
my-key-3

[networkimprov]

cc @mvdan

[mvdan]

Duplicate of #38105, where the fix is already in master and it's being backported for 1.14.3.