Fix wp_using_ext_object_cache() return type and add early-load guard.

Author: liaisontw

src/wp-includes/load.php

@@ -808,20 +808,31 @@ function wp_set_wpdb_vars() {
  * @return bool The current 'using' setting.
  */
 function wp_using_ext_object_cache( $using = null ) {
-    global $_wp_using_ext_object_cache;
+	global $_wp_using_ext_object_cache;
 
-    // Save the current state to return later.
-    $current_using = $_wp_using_ext_object_cache;
+	// Save the current state to return later.
+	$current_using = $_wp_using_ext_object_cache;
 
-    if ( null !== $using ) {
-        $_wp_using_ext_object_cache = (bool) $using;
-    }
+	if ( null !== $using ) {
+		$_wp_using_ext_object_cache = (bool) $using;
+	}
+
+	if ( null === $_wp_using_ext_object_cache ) {
+		if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
+			trigger_error(
+				sprintf(
+					'%1$s was called incorrectly. This function should not be called before the object cache is initialized. (This message was added in version 7.1.0.)',
+					__FUNCTION__
+				),
+				E_USER_NOTICE
+			);
+		}
+
+		// If the global is uninitialized, the value would be null, which violates the type signature.
+		return false;
+	}
 
-    /**
-     * Ensure the returned value is always a boolean. 
-     * If the global is uninitialized, it could be null, which violates the type signature.
-     */
-    return (bool) $current_using;
+	return (bool) $current_using;
 }
 
 /**