Description:
------------
Type hints in a derived class 
    that specify a function parameter 
        implements an interface 
            derived from the interface 
        specified in the (other) interface
implemented by the base class 
(whew!) 
are flagged as an error despite being type-compatible:

Fatal error: Declaration of TranslatedPlural::compose() must be compatible with that of Translatable::compose() in ...\TranslatedMessage.php on line 83


Reproduce code:
---------------
// Parameter type hint (BASE)
interface TranslationMap
{
	public	function fetch($the_key);
}

// Parameter type hint (DERIVED)
interface TranslationLexicon
	extends TranslationMap
{
	public function plural($the_noun, $the_count);
}

// Implementation Interface
interface Translatable
{
	function set_args();
	function compose(TranslationMap $map);
}

// Base class
class TranslatedMessage
	implements Translatable
{
	public function set_args()
	{
	}	

	public function compose(TranslationMap $lex)
	{
        }
}

// Derived class
class TranslatedPlural
	extends TranslatedMessage
{
    public function compose(TranslationLexicon $lex)
    {
    }
}


Expected result:
----------------
I expect that the derived class, TranslatedPlural, will be checked for a stricter (but type-compatible) interface than the base class.

Actual result:
--------------
Fatal error: Declaration of TranslatedPlural::compose() must be compatible with that of Translatable::compose() in ...\TranslatedMessage.php on line 83