go/printer: don't crash if AST contains BadXXX nodes

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5535048

src/pkg/go/printer/printer.go

@@ -773,8 +773,13 @@ func (p *printer) print(args ...interface{}) {
 				next = p.fset.Position(x) // accurate position of next item
 			}
 			tok = p.lastTok
+		case string:
+			// incorrect AST - print error message
+			data = x
+			isLit = true
+			tok = token.STRING
 		default:
-			fmt.Fprintf(os.Stderr, "print: unsupported argument type %T\n", f)
+			fmt.Fprintf(os.Stderr, "print: unsupported argument %v (%T)\n", f, f)
 			panic("go/printer type")
 		}
 		p.lastTok = tok

src/pkg/go/printer/printer_test.go

@@ -204,3 +204,18 @@ func init() {
 		panic("got " + s + ", want " + name)
 	}
 }
+
+// Verify that the printer doesn't crash if the AST contains BadXXX nodes.
+func TestBadNodes(t *testing.T) {
+	const src = "package p\n("
+	const res = "package p\nBadDecl\n"
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err == nil {
+		t.Errorf("expected illegal program")
+	}
+	var buf bytes.Buffer
+	Fprint(&buf, fset, f)
+	if buf.String() != res {
+		t.Errorf("got %q, expected %q", buf.String(), res)
+	}
+}