>< of parent/children class

OOP.php

<?php

declare(strict_types=1);

class A
{
}

class B extends A
{
}

class C extends B
{
}


class TestA
{
   public function x(C $c):A
   {
       return new C();
   }
}

class TestB extends TestA
{
   public function x(B $b):B
   {
       return new C();
   }
}

class TestC extends TestB
{
   public function x(A $a): C
   {
       return new C();
   }
}

$c = new C();

$testA = new TestA();
$testC = new TestC();


$resultA = $testA->x($c);
$resultC = $testC->x($c);

var_dump([
    'TestA::x(C): expect C' => get_class($resultA),
    'TestC::x(C): expect C' => get_class($resultC),
]);

$a = new A();
try {
    $resultA = $testA->x($a);
    echo "impossible";
} catch (TypeError $e){
    echo "TestA::x(A): expected TypeError\n";
}

echo 'TestC::x(A): expect C => '. get_class($testC->x($a));

https://3v4l.org/WV4jo

Output for 7.4.0 - 7.4.29, 8.0.1 - 8.0.19, 8.1.0 - 8.1.6
array(2) {
  ["TestA::x(C): expect C"]=>
  string(1) "C"
  ["TestC::x(C): expect C"]=>
  string(1) "C"
}
TestA::x(A): expected TypeError
TestC::x(A): expect C => C