Ensure $atts passed to $callback of add_shortcode() is always an array

[anonymized_14808221]

Problem

According to ​the documentation:

$callback callable Required
The callback function to run when the shortcode is found.
Every shortcode callback is passed three parameters by default, including an array of attributes ($atts), the shortcode content or null if not set ($content), and finally the shortcode tag itself ($shortcode_tag), in that order.

However, this part...

including an array of attributes ($atts)

...is not always true:

• if the shortcode is used without any attributes, then $atts is actually an empty string.
• apparently (according to code docbloc) it would also be a string of original arguments if it could not be parsed.

This discrepancy causes other dependant code to always fumble with the type of $atts, for example the ​source code of shortcode_atts has to typecasts $atts to array.
This would not be necessary, would $atts indeed be always an array from the start.

Further, in more complex programming scenarios where we assume $atts to be an array (as stated by the documentation), again we have to fumble with $atts, and in some cases, we cannot do anything but just accept that it may or may not be an array.
You can read more on that ​here

Cause

Thanks to the kind help of ​@bcworkz and further testing, I can confirm that the issue is within shortcode_parse_atts function - to be precise in the else statement here.
If the checks for if ( preg_match_all( $pattern, $text, $match, PREG_SET_ORDER ) ) fail, $atts is transformed to string using ltrim.

This makes thus poor sense to me:

1. First of all, the documentation mentions that $atts is an array. It never mentions other possible types.
2. It seem quite clear to me that $atts is always supposed and intended to be an array - never a string.
3. Even if the docblock for shortcode_parse_atts states that the returned value can be array or string (and if string, then the original arguments string if it cannot be parsed)... I do not see the actual reason or justification for this.

What is the usefulness of returning a string of arguments if it cannot be parsed or is empty?
It should then safely fail into an empty array, to keep consistency and not force dependant code to typecast $atts.

Proposal

Simply remove the else block in shortcode_parse_atts.
It is redundant and creates issues in particular coding structures + clear discrepancies in documentation.

Ensures $atts passed to $callback of add_shortcode() is always an array

[swissspidy]

Duplicate of #59249.

Committed in https://core.trac.wordpress.org/changeset/57597