expand: "$@" should expand to no fields when $# == 0

We had logic to always expand "..." to at least "", since that's how
double quotes in shell work. However, "$@" is an exception to that, so
it needs to be allowed to return zero fields.

Add a test case for the array version, too.

Fixes #473.

expand/expand.go

@@ -542,7 +542,6 @@ func (cfg *Config) wordFields(wps []syntax.WordPart) ([][]fieldPart, error) {
 			}
 			curField = append(curField, fp)
 		case *syntax.DblQuoted:
-			allowEmpty = true
 			if len(x.Parts) == 1 {
 				pe, _ := x.Parts[0].(*syntax.ParamExp)
 				if elems := cfg.quotedElems(pe); elems != nil {
@@ -558,6 +557,7 @@ func (cfg *Config) wordFields(wps []syntax.WordPart) ([][]fieldPart, error) {
 					continue
 				}
 			}
+			allowEmpty = true
 			wfield, err := cfg.wordField(x.Parts, quoteDouble)
 			if err != nil {
 				return nil, err

interp/interp_test.go

@@ -275,6 +275,10 @@ var runTests = []runTest{
 	{`echo -n "\\"`, `\`},
 	{`set -- a b c; x="$@"; echo "$x"`, "a b c\n"},
 	{`set -- b c; echo a"$@"d`, "ab cd\n"},
+	{`count() { echo $#; }; set --; count "$@"`, "0\n"},
+	{`count() { echo $#; }; set -- ""; count "$@"`, "1\n"},
+	{`count() { echo $#; }; a=(); count "${a[@]}"`, "0\n"},
+	{`count() { echo $#; }; a=(""); count "${a[@]}"`, "1\n"},
 	{`echo $1 $3; set -- a b c; echo $1 $3`, "\na c\n"},
 	{`[[ $0 == "bash" || $0 == "gosh" ]]`, ""},