output

$ hh_single_type_check test.php 
File "test.php", line 7, characters 7-7:
Class B does not initialize all of its members; name is not always initialized.
Make sure you systematically set $this->name when the method __construct is called.
Alternatively, you can define the member as optional (?...)
 (NastCheck[3015])

test.php

<?hh

abstract class A {
    protected string $name;
}

class B extends A {
    protected string $name = 'Hello';
}