Source

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

public function is_front_page() {
		// Most likely case.
		if ( 'posts' === get_option( 'show_on_front' ) && $this->is_home() ) {
			return true;
		} elseif ( 'page' === get_option( 'show_on_front' ) && get_option( 'page_on_front' )
			&& $this->is_page( get_option( 'page_on_front' ) )
		) {
			return true;
		} else {
			return false;
		}
	}