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Date: Wed, 26 Nov 2014 23:48:29 -0800
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To: Marc Bennewitz <dev@mabe.berlin>, internals@lists.php.net
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Subject: Re: [PHP-DEV] [RFC][Discussion] Return Type Variance Checking
From: smalyshev@gmail.com (Stanislav Malyshev)

Hi!

>> class FooFactory {
>>     function create(Foo $foo): Foo { return $foo; }
>> }
>>
>> class GooFactory extends FooFactory {
>>     function create(Goo $goo): Goo { return $goo; }
>> }
> OK HHVM allows it - we also allow it but trigger an E_STRICT error
> @see http://3v4l.org/UhtOb

This is because this code has LSP violation - if you have an object
about which you know it's typed as FooFactory, you should be able to
call it with any Foo object. But if this object is a GooFactory instead,
now not any Foo would serve, but only a subset of them - namely, Goo.
This clearly violates the principle "everything good for the parent must
be good for the child". Since PHP is a kind and nurturing language, we
only produce E_STRICT, some other languages would refuse to accept such
thing or interpret it as two different methods.
See also:
https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)#Covariant_method_argument_type

-- 
Stas Malyshev
smalyshev@gmail.com