x/exp/maps: Clone does not preserve nil

[icholy]

The x/exp/maps.Clone function doesn't preserve nil the way x/exp/slices.Clone does.

package main

import (
	"fmt"

	"golang.org/x/exp/maps"
)

func main() {
	var m map[string]int
	fmt.Printf("%#v\n", m)             // map[string]int(nil)
	fmt.Printf("%#v\n", maps.Clone(m)) // map[string]int{}
}

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

$ go version
go version go1.18.3 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/icholy/.cache/go-build"
GOENV="/home/icholy/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/icholy/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/icholy/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18.3"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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 -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build4044300034=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Cloned a nil map using x/exp/maps.Clone.

What did you expect to see?

The cloned map is also nil

What did you see instead?

The cloned map is an empty non-nil map.

[mknyszek]

CC @ianlancetaylor

[gopherbot]

Change https://go.dev/cl/417274 mentions this issue: x/exp/maps: Clone preserve nil in case it matters.