[PHP-BUG] Bug #67750 [NEW]: Subscript Operator claims empty brackets invalid for string

From:             ralph-bugsphpnet at inputplus dot co dot uk
Operating system: 
PHP version:      Irrelevant
Package:          PHP Language Specification
Bug Type:         Bug
Bug description:Subscript Operator claims empty brackets invalid for string

Description:
------------
https://github.com/php/php-langspec/blob/master/spec/10-expressions.md#subscript-operator
says

    *expression* can be omitted only if *subscript-expression* is used
in a
    modifiable-lvalue context and *postfix-expression* does not
designate a
    string.

Yet this is valid:

    $a =  ""; $a[] = "y"; var_dump($a);

I've omitted expression, postfix-expression is a string, just an empty
one.
The result is array("y");  the original empty string disappears.

See http://3v4l.org/vg7Oe for more.

Test script:
---------------
<?php
$a =  ""; $a[] = "y"; var_dump($a);
$b = "x"; $b[] = "y"; var_dump($b);


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Edit bug report at https://bugs.php.net/bug.php?id=67750&edit=1
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