gistfile1.txt

ckwalsh@ckwalsh:~$ cat foo.php 
<?hh

namespace Foo;

class Bar {
	function foobar(): this {
		return $this;
	}
}

$b = new Bar();
$b2 = $b->foobar();
ckwalsh@ckwalsh:~$ hhvm foo.php 

Fatal error: Value returned from Foo\Bar::foobar() must be of type Foo\this, Foo\Bar given in /home/ckwalsh/foo.php on line 7
ckwalsh@ckwalsh:~$