encoding/json: don't panic marshaling anonymous non-struct field

Add a check for this case and don't try to follow the anonymous
type's non-existent fields.

Fixes #4474.

R=rsc
CC=golang-dev
https://golang.org/cl/6945065

src/pkg/encoding/json/encode.go

@@ -617,13 +617,20 @@ func typeFields(t reflect.Type) []field {
 				index := make([]int, len(f.index)+1)
 				copy(index, f.index)
 				index[len(f.index)] = i
+
+				ft := sf.Type
+				if ft.Name() == "" && ft.Kind() == reflect.Ptr {
+					// Follow pointer.
+					ft = ft.Elem()
+				}
+
 				// Record found field and index sequence.
-				if name != "" || !sf.Anonymous {
+				if name != "" || !sf.Anonymous || ft.Kind() != reflect.Struct {
 					tagged := name != ""
 					if name == "" {
 						name = sf.Name
 					}
-					fields = append(fields, field{name, tagged, index, sf.Type,
+					fields = append(fields, field{name, tagged, index, ft,
 						opts.Contains("omitempty"), opts.Contains("string")})
 					if count[f.typ] > 1 {
 						// If there were multiple instances, add a second,
@@ -636,11 +643,6 @@ func typeFields(t reflect.Type) []field {
 				}
 
 				// Record new anonymous struct to explore in next round.
-				ft := sf.Type
-				if ft.Name() == "" {
-					// Must be pointer.
-					ft = ft.Elem()
-				}
 				nextCount[ft]++
 				if nextCount[ft] == 1 {
 					next = append(next, field{name: ft.Name(), index: index, typ: ft})

src/pkg/encoding/json/encode_test.go

@@ -186,3 +186,23 @@ func TestMarshalerEscaping(t *testing.T) {
 		t.Errorf("got %q, want %q", got, want)
 	}
 }
+
+type IntType int
+
+type MyStruct struct {
+	IntType
+}
+
+func TestAnonymousNonstruct(t *testing.T) {
+	var i IntType = 11
+	a := MyStruct{i}
+	const want = `{"IntType":11}`
+
+	b, err := Marshal(a)
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+	if got := string(b); got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+}