Newsgroups: php.internals
Path: news.php.net
Xref: news.php.net php.internals:108602
To: PHP Internals <internals@lists.php.net>
Message-ID: <16481aca-93f4-43f5-5ec2-413d19ade318@gmail.com>
Date: Sat, 15 Feb 2020 17:44:05 +0000
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101
 Thunderbird/68.5.0
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-GB
Subject: Allow null variables to be decremented
From: rowan.collins@gmail.com (Rowan Tommins)

Hi all,

There is currently an odd inconsistency when using the decrement 
operator on a null variable:

$a = null; $a=$a+1; // int(1)
$a = null; $a+=1; // int(1)
$a = null; ++$a; // int(1)

$a = null; $a=$a-1; // int(-1)
$a = null; $a-=1; // int(-1)
$a = null; --$a; // null

I would like to propose changing this behaviour for PHP 8, so that --$a 
would give int(-1), as I believe it is simply a long-standing bug.


This has been raised as a bug at least three times [1][2][3] but closed 
as documented behaviour / too much of a BC break. It is documented in 
the manual, but with no explanation of why it should work that way. [4]

I would be interested in any explanations of why it might be intended 
behaviour, or ways in which people might be relying on the current 
behaviour.

A proposal to change the behaviour was included in a wider RFC about 
standardising increment and decrement behaviour, but it never got beyond 
draft status. [5] I would prefer not to reopen that wider debate, but 
focus on this single issue.

As far as I can see, the change would be to add a "case IS_NULL" branch 
to decrement_function in zend_operators.c to match the one in 
increment_function. [6]


I will happily write up an RFC to formalise this, but wanted to gather 
people's immediate thoughts first.


Links:
[1] https://bugs.php.net/bug.php?id=20548
[2] https://bugs.php.net/bug.php?id=25674
[3] https://bugs.php.net/bug.php?id=41690
[4] https://www.php.net/manual/en/language.operators.increment.php
[5] https://wiki.php.net/rfc/normalize_inc_dec
[6] 
https://php-lxr.adamharvey.name/source/xref/master/Zend/zend_operators.c#2359


Regards,

-- 
Rowan Tommins (né Collins)
[IMSoP]