Add tests for count_user_posts behavior and caching scenarios

Introduce tests to validate count_user_posts correctness for non-existent users, users created after post assignment, and caching mechanisms. Ensure accurate caching for post type order variations and equivalence of string and array queries.

Feedback from @peterwilsoncc

tests/phpunit/tests/user/countUserPosts.php

@@ -89,4 +89,86 @@ public function test_count_user_posts_with_multiple_post_types() {
 	public function test_count_user_posts_should_ignore_non_existent_post_types() {
 		$this->assertSame( '4', count_user_posts( self::$user_id, array( 'foo', 'post' ) ) );
 	}
+
+	/**
+	 * User count should work for users that don't exist but have posts assigned.
+	 *
+	 * @ticket 39242
+	 */
+	public function test_count_user_posts_for_non_existent_user() {
+		$next_user_id = self::$user_id + 1;
+
+		// Assign post to next user.
+		self::factory()->post->create(
+			array(
+				'post_author' => $next_user_id,
+				'post_type'   => 'post',
+			)
+		);
+
+		$next_user_post_count = count_user_posts( $next_user_id );
+		$this->assertSame( '1', $next_user_post_count, 'Non-existent user is expected to have count of one post.' );
+	}
+
+	/**
+	 * Cached user count value should be accurate after user is created.
+	 *
+	 * @ticket 39242
+	 */
+	public function test_count_user_posts_for_user_created_after_being_assigned_posts() {
+		$next_user_id = self::$user_id + 1;
+
+		// Assign post to next user.
+		self::factory()->post->create(
+			array(
+				'post_author' => $next_user_id,
+				'post_type'   => 'post',
+			)
+		);
+
+		// Cache the user count.
+		count_user_posts( $next_user_id );
+
+		// Create user.
+		$real_next_user_id = self::factory()->user->create(
+			array(
+				'role' => 'author',
+			)
+		);
+
+		$this->assertSame( $next_user_id, $real_next_user_id, 'User ID should match calculated value' );
+		$this->assertSame( '1', count_user_posts( $next_user_id ), 'User is expected to have count of one post.' );
+	}
+
+	/**
+	 * User count cache should be hit regardless of post type order.
+	 *
+	 * @ticket 39242
+	 */
+	public function test_cache_should_be_hit_regardless_of_post_type_order() {
+		// Prime Cache
+		count_user_posts( self::$user_id, array( 'wptests_pt', 'post' ) );
+
+		$query_num_start = get_num_queries();
+		count_user_posts( self::$user_id, array( 'post', 'wptests_pt' ) );
+		$total_queries = get_num_queries() - $query_num_start;
+
+		$this->assertSame( 0, $total_queries );
+	}
+
+	/**
+	 * User count cache should be hit for string and array of post types.
+	 *
+	 * @ticket 39242
+	 */
+	public function test_cache_should_be_hit_for_string_and_array_equivalent_queries() {
+		// Prime Cache
+		count_user_posts( self::$user_id, 'post' );
+
+		$query_num_start = get_num_queries();
+		count_user_posts( self::$user_id, array( 'post' ) );
+		$total_queries = get_num_queries() - $query_num_start;
+
+		$this->assertSame( 0, $total_queries );
+	}
 }