encoding/json: fix panics on type mismatches.

Fixes #4222.
Fixes #4628.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/7100049

src/pkg/encoding/json/decode.go

@@ -347,15 +347,19 @@ func (d *decodeState) array(v reflect.Value) {
 
 	// Check type of target.
 	switch v.Kind() {
+	case reflect.Interface:
+		if v.NumMethod() == 0 {
+			// Decoding into nil interface?  Switch to non-reflect code.
+			v.Set(reflect.ValueOf(d.arrayInterface()))
+			return
+		}
+		// Otherwise it's invalid.
+		fallthrough
 	default:
 		d.saveError(&UnmarshalTypeError{"array", v.Type()})
 		d.off--
 		d.next()
 		return
-	case reflect.Interface:
-		// Decoding into nil interface?  Switch to non-reflect code.
-		v.Set(reflect.ValueOf(d.arrayInterface()))
-		return
 	case reflect.Array:
 	case reflect.Slice:
 		break
@@ -441,7 +445,7 @@ func (d *decodeState) object(v reflect.Value) {
 	v = pv
 
 	// Decoding into nil interface?  Switch to non-reflect code.
-	if v.Kind() == reflect.Interface {
+	if v.Kind() == reflect.Interface && v.NumMethod() == 0 {
 		v.Set(reflect.ValueOf(d.objectInterface()))
 		return
 	}
@@ -459,11 +463,9 @@ func (d *decodeState) object(v reflect.Value) {
 			v.Set(reflect.MakeMap(t))
 		}
 	case reflect.Struct:
+
 	default:
 		d.saveError(&UnmarshalTypeError{"object", v.Type()})
-	}
-
-	if !v.IsValid() {
 		d.off--
 		d.next() // skip over { } in input
 		return
@@ -646,7 +648,11 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool
 		case reflect.Bool:
 			v.SetBool(value)
 		case reflect.Interface:
-			v.Set(reflect.ValueOf(value))
+			if v.NumMethod() == 0 {
+				v.Set(reflect.ValueOf(value))
+			} else {
+				d.saveError(&UnmarshalTypeError{"bool", v.Type()})
+			}
 		}
 
 	case '"': // string
@@ -676,7 +682,11 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool
 		case reflect.String:
 			v.SetString(string(s))
 		case reflect.Interface:
-			v.Set(reflect.ValueOf(string(s)))
+			if v.NumMethod() == 0 {
+				v.Set(reflect.ValueOf(string(s)))
+			} else {
+				d.saveError(&UnmarshalTypeError{"string", v.Type()})
+			}
 		}
 
 	default: // number
@@ -705,6 +715,10 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool
 				d.saveError(err)
 				break
 			}
+			if v.NumMethod() != 0 {
+				d.saveError(&UnmarshalTypeError{"number", v.Type()})
+				break
+			}
 			v.Set(reflect.ValueOf(n))
 
 		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:

src/pkg/encoding/json/decode_test.go

@@ -1042,3 +1042,25 @@ func TestStringKind(t *testing.T) {
 	}
 
 }
+
+var decodeTypeErrorTests = []struct {
+	dest interface{}
+	src  string
+}{
+	{new(string), `{"user": "name"}`}, // issue 4628.
+	{new(error), `{}`},                // issue 4222
+	{new(error), `[]`},
+	{new(error), `""`},
+	{new(error), `123`},
+	{new(error), `true`},
+}
+
+func TestUnmarshalTypeError(t *testing.T) {
+	for _, item := range decodeTypeErrorTests {
+		err := Unmarshal([]byte(item.src), item.dest)
+		if _, ok := err.(*UnmarshalTypeError); !ok {
+			t.Errorf("expected type error for Unmarshal(%q, type %T): got %v instead",
+				item.src, item.dest, err)
+		}
+	}
+}