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Doc Bug #79484 Declared full covariance and contravariance support should be refined
Submitted: 2020-04-17 08:21 UTC Modified: 2020-04-17 14:07 UTC
Votes:1
Avg. Score:3.0 ± 0.0
Reproduced:1 of 1 (100.0%)
Same Version:0 (0.0%)
Same OS:0 (0.0%)
From: vlad dot turchinskiy at yandex dot ru Assigned:
Status: Not a bug Package: *General Issues
PHP Version: 7.4.5 OS: any
Private report: No CVE-ID: None
View Developer Edit
 [2020-04-17 08:21 UTC] vlad dot turchinskiy at yandex dot ru 
Description:
------------
On the doc page https://www.php.net/manual/en/language.oop5.variance.php declared that "As of PHP 7.4.0, full covariance and contravariance support was added".

As mentioned by derick@php.net at #79482 (https://bugs.php.net/bug.php?id=79482):
"Function arguments are contra-variant, not co-variant, meaning that an overloaded/inherited method can chose to accept a broader type. If it would allow for a more specific/narrow type, then an already existing bit of code could no longer call the overloaded with the same method arguments as the original parent 
class."

This should be refined in docs.

In the current state, docs goes against POLA (https://en.wikipedia.org/wiki/Principle_of_least_astonishment).

Provided test script to demonstrate.

Test script:
---------------
https://3v4l.org/vC0WM

Expected result:
----------------
The fact that "Function arguments are contra-variant, not co-variant" should be mentioned at https://www.php.net/manual/en/language.oop5.variance.php

Better, if it had an example.

Actual result:
--------------
Declared full covariance and contravariance support

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 [2020-04-17 08:53 UTC] cmb@php.net 
The PHP manual is already clear about that[1]:

| Covariance allows a child's method to return a more specific
| type than the return type of its parent's method. Whereas,
| contravariance allows a parameter type to be less specific in a
| child method, than that of its parent.

It appears, though, that the language specification is not[2].

[1] <https://www.php.net/manual/en/language.oop5.variance.php>
[2] <https://github.com/php/php-langspec/tree/master/spec>
 [2020-04-17 08:57 UTC] nikic@php.net
-Status: Open +Status: Not a bug
 [2020-04-17 08:57 UTC] nikic@php.net 
Can someone please remove the "PHP Language Specification" category on this bug tracker? php-langspec bug tracker is at https://github.com/php/php-langspec.
 [2020-04-17 09:00 UTC] vlad dot turchinskiy at yandex dot ru
-Package: PHP Language Specification +Package: *General Issues
 [2020-04-17 09:00 UTC] vlad dot turchinskiy at yandex dot ru 
Fixed category
 [2020-04-17 09:17 UTC] vlad dot turchinskiy at yandex dot ru 
cmb@php.net, > The PHP manual is already clear about that
I'm sorry for being annoying, but it isn't.

1) "full covariance and contravariance support" for me means that i can use both in return types and function parameter types. I can't, because of (2)
2) "Function arguments are contra-variant, not co-variant"
3) "contravariance allows a parameter type to be less specific in a child method, than that of its parent." is ok, but what about covariance in parameter type? Nothing.

Declared "full support" (1) becomes "partial" without any notice.

It is not clear from docs that "Function arguments are contra-variant, not co-variant".

This produces "what?!why?!" effect in situations like in test script (https://3v4l.org/vC0WM).
 [2020-04-17 09:32 UTC] nikic@php.net 
What kind of variance is supported for arguments and return values is dictated by the Liskov substitution principle. "Full" support here just means that variance respects all subtyping relationships, as supposed to only some of them, as was the case in earlier PHP versions.

Just to make this 100% clear: There will never be support for covariant argument types in PHP, because is simply is not sound.
 [2020-04-17 14:07 UTC] derick@php.net 
FWIW, I removed that category from the bug tracker now.
 
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