class MyClass implements A {}
new X_BASED(new MyClass);

won't be accepted while X says MyClass would be valid as a constructor Parameter -> contract not fullfilled. So PHP's error is correct.

hi johannes, please read better my example

interface A {}
interface X { public function __construct(A $x); }

class A_BASED implements A {}
class X_BASED implements X { public function __construct(A_BASED $TESTTEST){} }

interface X
says its constructor first parameter should be "instance of A"

XBASED implements X
says its constructor first parameter should be "instance of A_BASED"
A_BASED as type hint is correct because A_BASED implements A

interface X guarantees that any A may be passed. You limit it to A_BASED. MyClass is a valid A but no A_BASED, so X_BASED won't accept it, which conflicts with the promise by interface X.

but my A_BASED is a valid instance of A too

class A_BASED implements A {}


constructor

__construct(A_BASED $TESTTEST) {}

overrides

abstract __construct(A $TESTTEST) {}

and A_BASED instanceof A === TRUE

so i don't limit anything because all methods required by interface A are implemented in class A_BASED

i think it is a bug because with abstract classes works good

abstract class A {}
abstract class X { abstract public function __construct(A $x); } // must be at least A

class A_BASED extends A {}
class X_BASED extends X { public function __construct(A_BASED $x){} } // it is A_BASED, so it satisfy A

Currently, arguments are invariant. And if not, they should be contra-variant, but 
surely not co-variant like you want.

Your subclass is more strict that the super class which is wrong, as any subclass 
should be able to be used as instances of the super class.