encoding/json: don't panic on incorrect map argument

Fixes #8305.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/145680044

src/encoding/json/decode.go

@@ -445,7 +445,7 @@ func (d *decodeState) array(v reflect.Value) {
 }
 
 // object consumes an object from d.data[d.off-1:], decoding into the value v.
-// the first byte of the object ('{') has been read already.
+// the first byte ('{') of the object has been read already.
 func (d *decodeState) object(v reflect.Value) {
 	// Check for unmarshaler.
 	u, ut, pv := d.indirect(v, false)
@@ -478,7 +478,9 @@ func (d *decodeState) object(v reflect.Value) {
 		t := v.Type()
 		if t.Key().Kind() != reflect.String {
 			d.saveError(&UnmarshalTypeError{"object", v.Type()})
-			break
+			d.off--
+			d.next() // skip over { } in input
+			return
 		}
 		if v.IsNil() {
 			v.Set(reflect.MakeMap(t))

src/encoding/json/decode_test.go

@@ -406,6 +406,13 @@ var unmarshalTests = []unmarshalTest{
 		ptr: new(string),
 		out: "hello\ufffd\ufffd\ufffd\ufffd\ufffd\ufffdworld",
 	},
+
+	// issue 8305
+	{
+		in:  `{"2009-11-10T23:00:00Z": "hello world"}`,
+		ptr: &map[time.Time]string{},
+		err: &UnmarshalTypeError{"object", reflect.TypeOf(map[time.Time]string{})},
+	},
 }
 
 func TestMarshal(t *testing.T) {
@@ -514,6 +521,7 @@ func TestUnmarshal(t *testing.T) {
 		if tt.ptr == nil {
 			continue
 		}
+
 		// v = new(right-type)
 		v := reflect.New(reflect.TypeOf(tt.ptr).Elem())
 		dec := NewDecoder(bytes.NewReader(in))
@@ -521,7 +529,9 @@ func TestUnmarshal(t *testing.T) {
 			dec.UseNumber()
 		}
 		if err := dec.Decode(v.Interface()); !reflect.DeepEqual(err, tt.err) {
-			t.Errorf("#%d: %v want %v", i, err, tt.err)
+			t.Errorf("#%d: %v, want %v", i, err, tt.err)
+			continue
+		} else if err != nil {
 			continue
 		}
 		if !reflect.DeepEqual(v.Elem().Interface(), tt.out) {