From eb06a59f536692e9499b89abd3f156ff56d3075e Mon Sep 17 00:00:00 2001
From: Sergey Biryukov <sergeybiryukov@git.wordpress.org>
Date: Sun, 14 Jun 2020 21:40:10 +0000
Subject: [PATCH] Posts, Post Types: Introduce `default_category_post_types`
 filter.

The filter allows custom post types associated with the `category` taxonomy to opt in to requiring a default category, same as regular posts.

Props enrico.sorcinelli.
Fixes #43516.

git-svn-id: https://develop.svn.wordpress.org/trunk@48043 602fd350-edb4-49c9-b593-d223f7449a82
---
 src/wp-includes/post.php     | 24 ++++++++++++++++++++----
 tests/phpunit/tests/term.php | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+), 4 deletions(-)

diff --git a/src/wp-includes/post.php b/src/wp-includes/post.php
index 4384194092..6e12c072ba 100644
--- a/src/wp-includes/post.php
+++ b/src/wp-includes/post.php
@@ -4689,8 +4689,7 @@ function wp_set_post_terms( $post_id = 0, $tags = '', $taxonomy = 'post_tag', $a
 /**
  * Set categories for a post.
  *
- * If the post categories parameter is not set, then the default category is
- * going used.
+ * If no categories are provided, the default category is used.
  *
  * @since 2.1.0
  *
@@ -4706,10 +4705,27 @@ function wp_set_post_categories( $post_ID = 0, $post_categories = array(), $appe
 	$post_ID     = (int) $post_ID;
 	$post_type   = get_post_type( $post_ID );
 	$post_status = get_post_status( $post_ID );
-	// If $post_categories isn't already an array, make it one:
+
+	// If $post_categories isn't already an array, make it one.
 	$post_categories = (array) $post_categories;
+
 	if ( empty( $post_categories ) ) {
-		if ( 'post' === $post_type && 'auto-draft' !== $post_status ) {
+		/**
+		 * Filters post types (in addition to 'post') that require a default category.
+		 *
+		 * @since 5.5.0
+		 *
+		 * @param array $post_types An array of post types. Default empty array.
+		 */
+		$default_category_post_types = apply_filters( 'default_category_post_types', array() );
+
+		// Regular posts always require a default category.
+		$default_category_post_types = array_merge( $default_category_post_types, array( 'post' ) );
+
+		if ( in_array( $post_type, $default_category_post_types, true )
+			&& is_object_in_taxonomy( $post_type, 'category' )
+			&& 'auto-draft' !== $post_status
+		) {
 			$post_categories = array( get_option( 'default_category' ) );
 			$append          = false;
 		} else {
diff --git a/tests/phpunit/tests/term.php b/tests/phpunit/tests/term.php
index 90fdd59b60..08c08f8a7c 100644
--- a/tests/phpunit/tests/term.php
+++ b/tests/phpunit/tests/term.php
@@ -156,9 +156,11 @@ class Tests_Term extends WP_UnitTestCase {
 		$this->assertInternalType( 'array', $post->post_category );
 		$this->assertEquals( 1, count( $post->post_category ) );
 		$this->assertEquals( get_option( 'default_category' ), $post->post_category[0] );
+
 		$term1 = wp_insert_term( 'Foo', 'category' );
 		$term2 = wp_insert_term( 'Bar', 'category' );
 		$term3 = wp_insert_term( 'Baz', 'category' );
+
 		wp_set_post_categories( $post_id, array( $term1['term_id'], $term2['term_id'] ) );
 		$this->assertEquals( 2, count( $post->post_category ) );
 		$this->assertEquals( array( $term2['term_id'], $term1['term_id'] ), $post->post_category );
@@ -167,6 +169,7 @@ class Tests_Term extends WP_UnitTestCase {
 		$this->assertEquals( array( $term2['term_id'], $term3['term_id'], $term1['term_id'] ), $post->post_category );
 
 		$term4 = wp_insert_term( 'Burrito', 'category' );
+
 		wp_set_post_categories( $post_id, $term4['term_id'] );
 		$this->assertEquals( array( $term4['term_id'] ), $post->post_category );
 
@@ -182,6 +185,35 @@ class Tests_Term extends WP_UnitTestCase {
 		$this->assertEquals( get_option( 'default_category' ), $post->post_category[0] );
 	}
 
+	/**
+	 * @ticket 43516
+	 */
+	function test_wp_set_post_categories_sets_default_category_for_custom_post_types() {
+		add_filter( 'default_category_post_types', array( $this, 'filter_default_category_post_types' ) );
+
+		register_post_type( 'cpt', array( 'taxonomies' => array( 'category' ) ) );
+
+		$post_id = self::factory()->post->create( array( 'post_type' => 'cpt' ) );
+		$post    = get_post( $post_id );
+
+		$this->assertEquals( get_option( 'default_category' ), $post->post_category[0] );
+
+		$term = wp_insert_term( 'Foo', 'category' );
+
+		wp_set_post_categories( $post_id, $term['term_id'] );
+		$this->assertEquals( $term['term_id'], $post->post_category[0] );
+
+		wp_set_post_categories( $post_id, array() );
+		$this->assertEquals( get_option( 'default_category' ), $post->post_category[0] );
+
+		remove_filter( 'default_category_post_types', array( $this, 'filter_default_category_post_types' ) );
+	}
+
+	function filter_default_category_post_types( $post_types ) {
+		$post_types[] = 'cpt';
+		return $post_types;
+	}
+
 	/**
 	 * @ticket 25852
 	 */