os: make ExpandEnv recognize '-' as a special shell parameter

'-' is one of shell special parameters.

The existing implementation of isShellSpecialVar missed '-'
from the list, causing "$-" and "${-}" expand differently.

Fixes #16554

Change-Id: Iafc7984692cc83cff58f7c1e01267bf78b3a20a9
Reviewed-on: https://go-review.googlesource.com/25352
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/env.go

@@ -37,7 +37,7 @@ func ExpandEnv(s string) string {
 // shell variable such as $*.
 func isShellSpecialVar(c uint8) bool {
 	switch c {
-	case '*', '#', '$', '@', '!', '?', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
+	case '*', '#', '$', '@', '!', '?', '-', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
 		return true
 	}
 	return false

src/os/env_unix_test.go

@@ -7,6 +7,7 @@
 package os_test
 
 import (
+	"fmt"
 	. "os"
 	"testing"
 )
@@ -28,3 +29,28 @@ func TestSetenvUnixEinval(t *testing.T) {
 		}
 	}
 }
+
+var shellSpecialVarTests = []struct {
+	k, v string
+}{
+	{"*", "asterisk"},
+	{"#", "pound"},
+	{"$", "dollar"},
+	{"@", "at"},
+	{"!", "exclamation mark"},
+	{"?", "question mark"},
+	{"-", "dash"},
+}
+
+func TestExpandEnvShellSpecialVar(t *testing.T) {
+	for _, tt := range shellSpecialVarTests {
+		Setenv(tt.k, tt.v)
+		defer Unsetenv(tt.k)
+
+		argRaw := fmt.Sprintf("$%s", tt.k)
+		argWithBrace := fmt.Sprintf("${%s}", tt.k)
+		if gotRaw, gotBrace := ExpandEnv(argRaw), ExpandEnv(argWithBrace); gotRaw != gotBrace {
+			t.Errorf("ExpandEnv(%q) = %q, ExpandEnv(%q) = %q; expect them to be equal", argRaw, gotRaw, argWithBrace, gotBrace)
+		}
+	}
+}