How to show custom post type in all post section?

It seems pretty hacky, but this does work in my testing (I tested with built-in page post type):

// Change the post list and search query.
add_action( 'pre_get_posts', static function ( $query ) {
    if ( ! is_admin() || ! $query->is_main_query() ) {
        return;
    }

    if ( ! in_array( $query->get( 'post_type' ), array( 'post', 'Array' ) ) ) {
        return;
    }

    $query->set( 'post_type', array( 'post', 'page' ) );
} );

// If `post_type` is set to `Array`, change back to `post` to satisfy core validation.
add_action( 'admin_init', static function () {
    if ( ! isset( $_GET['s'] ) || '/wp-admin/edit.php' !== $_SERVER['PHP_SELF'] || ! isset( $_GET['post_type'] ) || 'Array' !== $_GET['post_type'] ) {
        return;
    }

    $_GET['post_type'] = 'post';
    $_REQUEST['post_type'] = 'post';
} );

The main functional piece here is checking the $_GET['post_type'] value on the admin_init action, and if it’s equal to Array, then change it to post (to satisfy WP core). Then when the query is actually constructed, change the post_type parameter to both post types.

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