Calling yii\db\Query addSelect($fields) overwrites default select set

What steps will reproduce the problem?

$query = ArClass::find();
$query->addSelect('extraField');
$item = $query->one();

What is the expected result?

$item has all default fields + 1 extraField, defined in addSelect()

What do you get instead?

$item has only extraField and does not have default fields.

Additional info

1. Due to docs, select property means "all by default" yii\db\Query:58

 /**
  * @var array the columns being selected. For example, `['id', 'name']`.
  * This is used to construct the SELECT clause in a SQL statement. If not set, it means selecting all columns.
  * @see select()
  */
public $select; 

2. Add statement ignores defauls if I did not defined defaults again. yii\db\Query:592

if ($this->select === null) {
    $this->select = $columns;
} else {
    $this->select = array_merge($this->select, $columns);
}

Some use cases:

In my opinion, query must select all default columns until I define columns set manually. Add meand adding element to some set. $select defined as all by default, but when you call addSelect() without calling select(), that means default not exists, add this.

See also #12249

The behavior is in framework/db/Query.php, especially the select property documentation and addSelect() logic around the referenced lines. Reproduce the PHP 7/Yii 2.0.13.1 example, review #12249 and the comment discussion, and verify that the final behavior preserves explicit select() cases while addressing the default-column case.