Index: wp-includes/general-template.php
===================================================================
--- wp-includes/general-template.php	(revision 31919)
+++ wp-includes/general-template.php	(working copy)
@@ -331,9 +331,10 @@
  *
  * @param string $redirect Path to redirect to on login.
  * @param bool $force_reauth Whether to force reauthorization, even if a cookie is present. Default is false.
+ * @param bool $interim_login Whether this is an interim login or not. Default is false.
  * @return string A log in URL.
  */
-function wp_login_url($redirect = '', $force_reauth = false) {
+function wp_login_url($redirect = '', $force_reauth = false, $interim_login = false) {
 	$login_url = site_url('wp-login.php', 'login');
 
 	if ( !empty($redirect) )
@@ -342,6 +343,9 @@
 	if ( $force_reauth )
 		$login_url = add_query_arg('reauth', '1', $login_url);
 
+	if ( $interim_login )
+		$login_url = add_query_arg( 'interim-login', '1', $login_url );
+
 	/**
 	 * Filter the login URL.
 	 *
@@ -348,11 +352,12 @@
 	 * @since 2.8.0
 	 * @since 4.2.0 The `$force_reauth` parameter was added.
 	 *
-	 * @param string $login_url    The login URL.
-	 * @param string $redirect     The path to redirect to on login, if supplied.
-	 * @param bool   $force_reauth Whether to force reauthorization, even if a cookie is present.
+	 * @param string $login_url     The login URL.
+	 * @param string $redirect      The path to redirect to on login, if supplied.
+	 * @param bool   $force_reauth  Whether to force reauthorization, even if a cookie is present.
+	 * @param bool   $interim_login Whether this is an interim login or not.
 	 */
-	return apply_filters( 'login_url', $login_url, $redirect, $force_reauth );
+	return apply_filters( 'login_url', $login_url, $redirect, $force_reauth, $interim_login );
 }
 
 /**
Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 31919)
+++ wp-includes/functions.php	(working copy)
@@ -4645,7 +4645,7 @@
  * @since 3.6.0
  */
 function wp_auth_check_html() {
-	$login_url = wp_login_url();
+	$login_url = wp_login_url( '', false, true );
 	$current_domain = ( is_ssl() ? 'https://' : 'http://' ) . $_SERVER['HTTP_HOST'];
 	$same_domain = ( strpos( $login_url, $current_domain ) === 0 );
 
@@ -4668,7 +4668,7 @@
 
 	if ( $same_domain ) {
 		?>
-		<div id="wp-auth-check-form" data-src="<?php echo esc_url( add_query_arg( array( 'interim-login' => 1 ), $login_url ) ); ?>"></div>
+		<div id="wp-auth-check-form" data-src="<?php echo esc_url( $login_url ); ?>"></div>
 		<?php
 	}