Administration: Relax the default value check for the `$position` argument added to `add_submenu_page()` and related functions in [46197].

Due to a confusion with `add_menu_page()`, which takes the `$icon_url` parameter, while `add_submenu_page()` does not, some plugins were passing in a string instead of integer as `$position`, causing backward compatibility issues.

A `_doing_it_wrong()` message is now added to alert developers of the wrong parameter type.

Props david.binda, desrosj, 123host, dennis_f, MattyRob.
Reviewed by desrosj.
Fixes #48249.
Built from https://develop.svn.wordpress.org/trunk@46570


git-svn-id: http://core.svn.wordpress.org/trunk@46367 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/plugin.php

@@ -1373,7 +1373,19 @@ function add_submenu_page( $parent_slug, $page_title, $menu_title, $capability,
 	}
 
 	$new_sub_menu = array( $menu_title, $capability, $menu_slug, $page_title );
-	if ( null === $position ) {
+	if ( ! is_int( $position ) ) {
+		if ( null !== $position ) {
+			_doing_it_wrong(
+				__FUNCTION__,
+				sprintf(
+					/* translators: %s: add_submenu_page() */
+					__( 'The seventh parameter passed to %s should be an integer representing menu position.' ),
+					'<code>add_submenu_page()</code>'
+				),
+				'5.3.0'
+			);
+		}
+
 		$submenu[ $parent_slug ][] = $new_sub_menu;
 	} else {
 		// If position is equal or higher than the number of items in the array, append the submenu.

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.3-RC1-46569';
+$wp_version = '5.3-RC1-46570';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.