expand: tilde expansions shouldn't expand further

To quote the latest version of the POSIX Shell spec:

	The pathname resulting from tilde expansion shall be treated as if
	quoted to prevent it being altered by field splitting and pathname
	expansion.

That is, when HOME='/*', the command "echo ~" should just print "/*" and
not do any globbing at all.

This was usually not a problem on Unix-like systems, but Windows uses
backslashes as path separators. Thus, HOME='C:\foo\bar' is a very normal
occurrence.

However, since we treated the result of tilde expansion as unquoted,
"echo ~" would result in the output "C:foobar", since we'd eat the
backslashes when taking care of character escapes.

Fix that, and add a few tests for these edge cases.

Fixes mvdan#356.

expand/expand.go

@@ -414,7 +414,11 @@ func (cfg *Config) wordField(wps []syntax.WordPart, ql quoteLevel) ([]fieldPart,
 		case *syntax.Lit:
 			s := x.Value
 			if i == 0 && ql == quoteNone {
-				s = cfg.expandUser(s)
+				if prefix, rest := cfg.expandUser(s); prefix != "" {
+					// TODO: return two separate fieldParts,
+					// like in wordFields?
+					s = prefix + rest
+				}
 			}
 			if ql == quoteDouble && strings.Contains(s, "\\") {
 				buf := cfg.strBuilder()
@@ -509,7 +513,12 @@ func (cfg *Config) wordFields(wps []syntax.WordPart) ([][]fieldPart, error) {
 		case *syntax.Lit:
 			s := x.Value
 			if i == 0 {
-				s = cfg.expandUser(s)
+				prefix, rest := cfg.expandUser(s)
+				curField = append(curField, fieldPart{
+					quote: quoteSingle,
+					val:   prefix,
+				})
+				s = rest
 			}
 			if strings.Contains(s, "\\") {
 				buf := cfg.strBuilder()
@@ -603,28 +612,27 @@ func (cfg *Config) quotedElems(pe *syntax.ParamExp) []string {
 	return nil
 }
 
-func (cfg *Config) expandUser(field string) string {
+func (cfg *Config) expandUser(field string) (prefix, rest string) {
 	if len(field) == 0 || field[0] != '~' {
-		return field
+		return "", field
 	}
 	name := field[1:]
-	rest := ""
 	if i := strings.Index(name, "/"); i >= 0 {
 		rest = name[i:]
 		name = name[:i]
 	}
 	if name == "" {
-		return cfg.Env.Get("HOME").String() + rest
+		return cfg.Env.Get("HOME").String(), rest
 	}
 	if vr := cfg.Env.Get("HOME " + name); vr.IsSet() {
-		return vr.String() + rest
+		return vr.String(), rest
 	}
 
 	u, err := user.Lookup(name)
 	if err != nil {
-		return field
+		return "", field
 	}
-	return u.HomeDir + rest
+	return u.HomeDir, rest
 }
 
 func findAllIndex(pattern, name string, n int) [][]int {

interp/interp_test.go

@@ -655,6 +655,12 @@ var runTests = []runTest{
 	{`[[ 'ab\c' == *\\* ]]`, ""},
 	{`[[ foo/bar == foo* ]]`, ""},
 	{"[[ a == [ab ]]", "exit status 1"},
+	{`HOME='/*'; echo ~; echo "$HOME"`, "/*\n/*\n"},
+	{`test -d ~`, ""},
+	{`foo=~; test -d $foo`, ""},
+	{`foo=~; test -d "$foo"`, ""},
+	{`foo='~'; test -d $foo`, "exit status 1"},
+	{`foo='~'; [ $foo == '~' ]`, ""},
 	{
 		`[[ ~ == "$HOME" ]] && [[ ~/foo == "$HOME/foo" ]]`,
 		"",

interp/test.go

@@ -20,7 +20,7 @@ import (
 func (r *Runner) bashTest(ctx context.Context, expr syntax.TestExpr, classic bool) string {
 	switch x := expr.(type) {
 	case *syntax.Word:
-		return r.literal(x)
+		return r.document(x)
 	case *syntax.ParenTest:
 		return r.bashTest(ctx, x.X, classic)
 	case *syntax.BinaryTest: