Index: src/wp-includes/widgets.php
===================================================================
--- src/wp-includes/widgets.php	(revision 35846)
+++ src/wp-includes/widgets.php	(working copy)
@@ -617,6 +617,19 @@
 	}
 
 	$sidebars_widgets = wp_get_sidebars_widgets();
+
+	/**
+	 * Filter the index (sidebar name) before generating the widget area.
+	 *
+	 * @since 4.5.0
+	 *
+	 * @param string $index                  The name of the sidebar which is going to be generated.
+	 * @param array  $wp_registered_sidebars Defined widget areas.
+	 * @param array  $wp_registered_widgets  All the widgets user has created.
+	 * @param array  $sidebars_widgets       List of inactive widgets and all widget areas with their allocated widgets.
+	 */
+	$index = apply_filters( 'dynamic_sidebar_index', $index, $wp_registered_sidebars, $wp_registered_widgets, $sidebars_widgets );
+
 	if ( empty( $wp_registered_sidebars[ $index ] ) || empty( $sidebars_widgets[ $index ] ) || ! is_array( $sidebars_widgets[ $index ] ) ) {
 		/** This action is documented in wp-includes/widget.php */
 		do_action( 'dynamic_sidebar_before', $index, false );