encoding/gob: don't send type info for unexported fields

Fixes #2517.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5440079

src/pkg/encoding/gob/encoder.go

@@ -119,7 +119,9 @@ func (enc *Encoder) sendActualType(w io.Writer, state *encoderState, ut *userTyp
 	switch st := actual; st.Kind() {
 	case reflect.Struct:
 		for i := 0; i < st.NumField(); i++ {
-			enc.sendType(w, state, st.Field(i).Type)
+			if isExported(st.Field(i).Name) {
+				enc.sendType(w, state, st.Field(i).Type)
+			}
 		}
 	case reflect.Array, reflect.Slice:
 		enc.sendType(w, state, st.Elem())

src/pkg/encoding/gob/encoder_test.go

@@ -662,3 +662,19 @@ func TestSequentialDecoder(t *testing.T) {
 		}
 	}
 }
+
+// Should be able to have unrepresentable fields (chan, func) as long as they
+// are unexported.
+type Bug2 struct {
+	A int
+	b chan int
+}
+
+func TestUnexportedChan(t *testing.T) {
+	b := Bug2{23, make(chan int)}
+	var stream bytes.Buffer
+	enc := NewEncoder(&stream)
+	if err := enc.Encode(b); err != nil {
+		t.Fatalf("error encoding unexported channel: %s", err)
+	}
+}