Source

File: wp-includes/class-wp-post.php

public function __get( $key ) {
		if ( 'page_template' === $key && $this->__isset( $key ) ) {
			return get_post_meta( $this->ID, '_wp_page_template', true );
		}

		if ( 'post_category' === $key ) {
			if ( is_object_in_taxonomy( $this->post_type, 'category' ) ) {
				$terms = get_the_terms( $this, 'category' );
			}

			if ( empty( $terms ) ) {
				return array();
			}

			return wp_list_pluck( $terms, 'term_id' );
		}

		if ( 'tags_input' === $key ) {
			if ( is_object_in_taxonomy( $this->post_type, 'post_tag' ) ) {
				$terms = get_the_terms( $this, 'post_tag' );
			}

			if ( empty( $terms ) ) {
				return array();
			}

			return wp_list_pluck( $terms, 'name' );
		}

		// Rest of the values need filtering.
		if ( 'ancestors' === $key ) {
			$value = get_post_ancestors( $this );
		} else {
			$value = get_post_meta( $this->ID, $key, true );
		}

		if ( $this->filter ) {
			$value = sanitize_post_field( $key, $value, $this->ID, $this->filter );
		}

		return $value;
	}