Merge sub properties in add_post_type_support().

[fabiankaegy]

Previously, calling add_post_type_support() multiple times for the same post type and feature with array arguments would overwrite the previous arguments instead of merging them. This prevented plugins and themes from incrementally adding sub-properties to the same feature.

This changes the behavior to merge associative array arguments when the function is called multiple times with the same post type and feature combination.

Example usage:

add_post_type_support( 'page', 'editor', array(
    'default-mode' => 'template-locked',
) );

add_post_type_support( 'page', 'editor', array(
    'block-comments' => true,
) );

// Both properties are now preserved instead of only the last one.

The fix maintains full backwards compatibility by only merging when both the existing and new values are associative arrays. All other calling patterns continue to work as before.

Props @fabiankaegy, @Mamaduka, @t-hamano .

Trac ticket: https://core.trac.wordpress.org/ticket/64156#ticket

---

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Core Committers: Use this line as a base for the props when committing in SVN:

Props fabiankaegy, adamsilverstein, mamaduka.

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[github-actions]

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[adamsilverstein]

Nice!

[Mamaduka]

IMO, the current behavior is broken, but given everything WP, would this be considered a breaking change?

cc @desrosj, @peterwilsoncc

[westonruter]

Nit: This is a bit easier to read and I think is more common formatting for multi-line conditions:

Suggested change

	if ( isset( $_wp_post_type_features[ $post_type ][ $feature ][0] )
	&& is_array( $_wp_post_type_features[ $post_type ][ $feature ][0] )
	&& isset( $args[0] )
	&& is_array( $args[0] ) ) {
	if (
	isset( $_wp_post_type_features[ $post_type ][ $feature ][0] ) &&
	is_array( $_wp_post_type_features[ $post_type ][ $feature ][0] ) &&
	isset( $args[0] ) &&
	is_array( $args[0] )
	) {

[westonruter]

The is_array( $args[0] ) call here isn't checking whether the arg is an associative array. Shouldn't it also check if ! isset( $args[0][0] )?

[westonruter]

Or rather: ! array_is_list( $args[0] )

[westonruter]

Additionally, what if someone tries to merge an empty array? Should that be allowed? So perhaps the new check here should be: ! array_is_list( $args[0] ) || empty( $args[0] )

[westonruter]

Per below, should this check to make sure it is actually an associative array? I think it should add this check: ! array_is_list( $_wp_post_type_features[ $post_type ][ $feature ][0] )