Bug in go/parser when coverting identifier lists.

Fixes #567.

R=rsc
CC=golang-dev
https://golang.org/cl/195041

src/pkg/go/parser/parser.go

@@ -464,7 +464,7 @@ func (p *parser) makeIdentList(list *vector.Vector) []*ast.Ident {
 		if !isIdent {
 			pos := x.(ast.Expr).Pos()
 			p.errorExpected(pos, "identifier")
-			idents[i] = &ast.Ident{pos, ast.NewObj(ast.Err, pos, "")}
+			ident = &ast.Ident{pos, ast.NewObj(ast.Err, pos, "_")}
 		}
 		idents[i] = ident
 	}

src/pkg/go/parser/parser_test.go

@@ -34,6 +34,7 @@ var validPrograms = []interface{}{
 	`package main; func main() { if f(T{}) {} }` + "\n",
 	`package main; func main() { _ = (<-chan int)(x) }` + "\n",
 	`package main; func main() { _ = (<-chan <-chan int)(x) }` + "\n",
+	`package main; func f(func() (func() int))` + "\n",
 }