encoding/json: don't marshal unexported embedded fields of non struct type

Marshal must process unexported embedded fields of struct type,
looking for exported fields in those structs. However, it must
not process unexported embedded fields of non-struct type.

For example, consider:

    type t1 struct {
        X int
    }
    type t2 int
    type T struct {
        t1
        t2
    }

When considering T, Marshal must process t1 to find t1.X.
Marshal must not process t2, but it was. Fix that.

Fixes #18009

Change-Id: I62ba0b65ba30fd927990e101a26405a9998787a3
Reviewed-on: https://go-review.googlesource.com/33773
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/encoding/json/encode.go

@@ -1093,7 +1093,7 @@ func typeFields(t reflect.Type) []field {
 			// Scan f.typ for fields to include.
 			for i := 0; i < f.typ.NumField(); i++ {
 				sf := f.typ.Field(i)
-				if sf.PkgPath != "" && !sf.Anonymous { // unexported
+				if sf.PkgPath != "" && (!sf.Anonymous || sf.Type.Kind() != reflect.Struct) { // unexported
 					continue
 				}
 				tag := sf.Tag.Get("json")

src/encoding/json/encode_test.go

@@ -273,6 +273,59 @@ func TestAnonymousNonstruct(t *testing.T) {
 	}
 }
 
+type unexportedIntType int
+
+type MyStructWithUnexportedIntType struct {
+	unexportedIntType
+}
+
+func TestAnonymousNonstructWithUnexportedType(t *testing.T) {
+	a := MyStructWithUnexportedIntType{11}
+	const want = `{}`
+
+	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)
+	}
+}
+
+type MyStructContainingUnexportedStruct struct {
+	unexportedStructType1
+	unexportedIntType
+}
+
+type unexportedStructType1 struct {
+	ExportedIntType1
+	unexportedIntType
+	unexportedStructType2
+}
+
+type unexportedStructType2 struct {
+	ExportedIntType2
+	unexportedIntType
+}
+
+type ExportedIntType1 int
+type ExportedIntType2 int
+
+func TestUnexportedAnonymousStructWithExportedType(t *testing.T) {
+	s2 := unexportedStructType2{3, 4}
+	s1 := unexportedStructType1{1, 2, s2}
+	a := MyStructContainingUnexportedStruct{s1, 6}
+	const want = `{"ExportedIntType1":1,"ExportedIntType2":3}`
+
+	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)
+	}
+}
+
 type BugA struct {
 	S string
 }