Use static:: instead of self:: to support late static bindings

Author: jokeputs

app/code/Magento/Sales/Model/Order/Creditmemo.php

@@ -439,14 +439,14 @@ public function canVoid()
      */
     public static function getStates()
     {
-        if (is_null(self::$_states)) {
-            self::$_states = [
+        if (is_null(static::$_states)) {
+            static::$_states = [
                 self::STATE_OPEN => __('Pending'),
                 self::STATE_REFUNDED => __('Refunded'),
                 self::STATE_CANCELED => __('Canceled'),
             ];
         }
-        return self::$_states;
+        return static::$_states;
     }
 
     /**
@@ -461,11 +461,11 @@ public function getStateName($stateId = null)
             $stateId = $this->getState();
         }
 
-        if (is_null(self::$_states)) {
-            self::getStates();
+        if (is_null(static::$_states)) {
+            static::getStates();
         }
-        if (isset(self::$_states[$stateId])) {
-            return self::$_states[$stateId];
+        if (isset(static::$_states[$stateId])) {
+            return static::$_states[$stateId];
         }
         return __('Unknown State');
     }

app/code/Magento/Sales/Model/Order/Invoice.php

@@ -543,14 +543,14 @@ public function addItem(\Magento\Sales\Model\Order\Invoice\Item $item)
      */
     public static function getStates()
     {
-        if (null === self::$_states) {
-            self::$_states = [
+        if (null === static::$_states) {
+            static::$_states = [
                 self::STATE_OPEN => __('Pending'),
                 self::STATE_PAID => __('Paid'),
                 self::STATE_CANCELED => __('Canceled'),
             ];
         }
-        return self::$_states;
+        return static::$_states;
     }
 
     /**
@@ -565,11 +565,11 @@ public function getStateName($stateId = null)
             $stateId = $this->getState();
         }
 
-        if (null === self::$_states) {
-            self::getStates();
+        if (null === static::$_states) {
+            static::getStates();
         }
-        if (isset(self::$_states[$stateId])) {
-            return self::$_states[$stateId];
+        if (isset(static::$_states[$stateId])) {
+            return static::$_states[$stateId];
         }
         return __('Unknown State');
     }