Plugins: Restore backward compatibility for PHP4-style passing of `array( &$this )` as action argument to `do_action()`.

This is a follow-up to [46149] to avoid unnecessary breakage in case of using the old notation.

Props david.binda, jrf.
Reviewed by azaozz.
Fixes #48312.
Built from https://develop.svn.wordpress.org/trunk@46568


git-svn-id: http://core.svn.wordpress.org/trunk@46365 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/plugin.php

@@ -470,6 +470,9 @@ function do_action( $tag, ...$arg ) {
 
 	if ( empty( $arg ) ) {
 		$arg[] = '';
+	} elseif ( is_array( $arg[0] ) && 1 === count( $arg[0] ) && isset( $arg[0][0] ) && is_object( $arg[0][0] ) ) {
+		// Backward compatibility for PHP4-style passing of `array( &$this )` as action `$arg`.
+		$arg[0] = $arg[0][0];
 	}
 
 	$wp_filter[ $tag ]->do_action( $arg );

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.3-RC1-46567';
+$wp_version = '5.3-RC1-46568';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.