encoding/json: fix unmarshal invalid json.Number

Unmarshal of invalid values, that is a value that is not a number,
into fields of type json.Number should fail.

Fixes #14702

src/encoding/json/decode.go

@@ -950,6 +950,9 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool
 			v.SetBytes(b[:n])
 		case reflect.String:
 			v.SetString(string(s))
+			if v.Type() == numberType && !isValidNumber(string(s)) {
+				return fmt.Errorf("json: invalid number literal, trying to unmarshal %q into Number", item)
+			}
 		case reflect.Interface:
 			if v.NumMethod() == 0 {
 				v.Set(reflect.ValueOf(string(s)))

src/encoding/json/decode_test.go

@@ -1192,6 +1192,37 @@ func TestNumberAccessors(t *testing.T) {
 	}
 }
 
+// golang.org/issue/14702
+func TestUnmarshalNumber(t *testing.T) {
+	var b Number
+	if err := Unmarshal([]byte(`invalid`), &b); err == nil {
+		t.Error("Unmarshal of invalid value into json.Number did not fail.")
+	}
+	if err := Unmarshal([]byte(`"invalid"`), &b); err == nil {
+		t.Error("Unmarshal of invalid value into json.Number did not fail.")
+	}
+
+	validJson := `{"A":"invalid"}`
+
+	var test struct {
+		A Number
+	}
+	if err := Unmarshal([]byte(validJson), &test); err == nil {
+		t.Error("Unmarshal of invalid value into struct field of type json.Number did not fail.")
+	}
+	var testString struct {
+		A Number `json:",string"`
+	}
+	if err := Unmarshal([]byte(validJson), &testString); err == nil {
+		t.Error("Unmarshal of invalid value into struct field of type json.Number with string option did not fail.")
+	}
+
+	var testMap map[string]Number
+	if err := Unmarshal([]byte(validJson), &testMap); err == nil {
+		t.Error("Unmarshal of invalid value into map[string]json.Number did not fail.")
+	}
+}
+
 func TestLargeByteSlice(t *testing.T) {
 	s0 := make([]byte, 2000)
 	for i := range s0 {