Description:
------------
PDO::FETCH_CLASSTYPE/PDO::FETCH_SERIALIZE can only be used together with PDO::FETCH_CLASS

https://github.com/php/php-src/blob/php-7.2.3/ext/pdo/pdo_stmt.c#L1225

it should be throw exception, but

1. if use PDOStatement::setFetchMode(), will throw exception
2. if use PDOStatement::fetch(), fetch type will reset to PDO::ATTR_DEFAULT_FETCH_MODE
3. if use PDOStatement::fetchAll(), fetch type will reset to PDO::FETCH_BOTH 



Test script:
---------------
$statement = $pdo->prepare("SELECT * FROM foo");
$statement->execute([]);

// will throw exception
$statement->setFetchMode(PDO::FETCH_CLASSTYPE);
$row = $statement->fetch();


// fetch type reset to PDO::ATTR_DEFAULT_FETCH_MODE
$row = $statement->fetch(PDO::FETCH_CLASSTYPE);

// fetch type reset to PDO::FETCH_BOTH 
$lists = $statement->fetchAll(PDO::FETCH_CLASSTYPE);

Expected result:
----------------
see test script

Actual result:
--------------
I think all these three method should throw exception, but it's not

it's a Feature ?