os: ExpandEnv: curly brackets not matched correctly

[sding3]

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

go version go1.14.4 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/shang/.cache/go-build"
GOENV="/home/shang/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/shang/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/shang/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"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build728725462=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Ran the following code:

package main

import (
	"fmt"
	"os"
)

func main() {
	s := `${${foo}}`
	handleError(os.Setenv("HOME", "~"))
	handleError(os.Setenv("foo", "HOME"))
	handleError(os.Setenv("{foo}", "bar"))
	handleError(os.Setenv("{${foo}}", "baz"))
	fmt.Println(os.ExpandEnv(s))
}

func handleError(e error) {
	if e != nil {
		panic(e)
	}
}

What did you expect to see?

I expect to see any one of ~,${HOME},${bar}, the empty string, or baz

What did you see instead?

}

---

Slightly related: #35752

[dmitshur]

/cc @ianlancetaylor @bradfitz per owners.

[ianlancetaylor]

This is working as documented. os.ExpandEnv is not the shell, it's just a simple one level expander. In the string ${${foo}}, it replaces ${var} with the value of var in the environment. In this case var is ${foo. That is the string that appears from ${ to the next }. That string has no value in the environment, so it is replaced with the empty string. That leaves just the trailing }, which is what you see.

[vatine]

This is also (effectively) what bash does:

test$ echo $foo
bar
test$ echo $bar
gazonk
test$ echo ${${foo}}
bash: ${${foo}}: bad substitution