PHP notice when a post has a term from a "numeric" taxonomy

[strategio]

This happens when the taxonomy name is numerical like "123456" for instance.

Steps to reproduce:

• After WP installation, add the following snippet to the theme's functions.php.

<?php
function wpmltm_2077_create_taxonomy() {
        $labels = array(
                'name'          => _x( '123456', 'taxonomy general name', 'textdomain' ),
                'singular_name' => _x( '123456', 'taxonomy singular name', 'textdomain' ),
        );

        $args = array(
                'hierarchical'      => true,
                'labels'            => $labels,
                'show_ui'           => true,
                'show_admin_column' => true,
                'query_var'         => true,
                'rewrite'           => array( 'slug' => 'xyz' ),
        );

        register_taxonomy( '123456', array( 'post' ), $args );
}
add_action( 'init', 'wpmltm_2077_create_taxonomy' );

• Go to Posts -> 123456 and add a new term "Test term".
• Add a new post with the term "Test term" for taxonomy 123456 and click on "Publish".
• Go to Posts

=> The PHP notice below is thrown.

( ! ) Notice: Trying to get property of non-object in wp-includes\post.php on line 6161
Call Stack
#	Time	Memory	Function	Location
1	0.0057	466920	{main}( )	...\edit.php:0
2	0.4055	25649896	WP_Posts_List_Table->prepare_items( )	...\edit.php:180
3	0.4055	25649968	wp_edit_posts_query( ??? )	...\class-wp-posts-list-table.php:136
4	0.4069	25658096	wp( ??? )	...\post.php:1073
5	0.4069	25658120	WP->main( ??? )	...\functions.php:960
6	0.4097	25683376	WP->query_posts( )	...\class-wp.php:715
7	0.4097	25683440	WP_Query->query( ??? )	...\class-wp.php:599
8	0.4097	25683384	WP_Query->get_posts( )	...\class-wp-query.php:3222
9	0.4234	25851048	wp_queue_posts_for_term_meta_lazyload( ??? )	...\class-wp-query.php:3002

*Debugging:*

• The actual value of $term at line 6161 is null.
• The root cause is in WP_Term::get_instance where we have a strict comparison between the taxonomy name. On one side, we have an integer and on the other side we have a numerical string.

This is because the taxonomies are stored in an array and the key is the taxonomy slug (which can be numerical).

Here's PHP doc ​http://php.net/manual/en/language.types.array.php:

Strings containing valid decimal integers, unless the number is preceded by a + sign, will be cast to the integer type. E.g. the key "8" will actually be stored under 8. On the other hand "08" will not be cast, as it isn't a valid decimal integer.

[strategio]

I had a look at the documentation and now I am not sure this is really an issue.

It says:

Name should only contain lowercase letters and the underscore character, and not be more than 32 characters long (database structure restriction).

Does this mean we cannot use numbers in the taxonomy name?

Note: We detected this issue because we had a failing test and it turned out because of a numeric taxonomy name. I am not sure this is likely to happen in the real world.

[dd32]

I don't think using a numeric taxonomy slug is going to be expected or run into in real world usage.

We could forbid numeric taxonomy slugs (like we do with empty, or >32char ones), but I'm not sure that's worth actual effort.

I'm going to close this as wontfix but other committers are free to re-open it.