Resolves 6731 by adding a custom message for refunding shipping

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

@@ -10,6 +10,7 @@ class Discount extends AbstractTotal
     /**
      * @param \Magento\Sales\Model\Order\Creditmemo $creditmemo
      * @return $this
+     * @throws \Magento\Framework\Exception\LocalizedException
      */
     public function collect(\Magento\Sales\Model\Order\Creditmemo $creditmemo)
     {
@@ -26,6 +27,16 @@ public function collect(\Magento\Sales\Model\Order\Creditmemo $creditmemo)
          * basing on how much shipping should be refunded.
          */
         $baseShippingAmount = $this->getBaseShippingAmount($creditmemo);
+
+        /**
+         * If credit memo's shipping amount is set and Order's shipping amount is 0,
+         * throw exception with different message
+         */
+        if ($baseShippingAmount && $order->getBaseShippingAmount() <= 0) {
+            throw new \Magento\Framework\Exception\LocalizedException(
+                new \Magento\Framework\Phrase("You can not refund shipping if there is no shipping amount.", [])
+            );
+        }
         if ($baseShippingAmount) {
             $baseShippingDiscount = $baseShippingAmount *
                 $order->getBaseShippingDiscountAmount() /

app/code/Magento/Sales/Test/Unit/Model/Order/Creditmemo/Total/DiscountTest.php

@@ -74,7 +74,7 @@ public function testCollect()
         $this->orderMock->expects($this->once())
             ->method('getBaseShippingDiscountAmount')
             ->willReturn(1);
-        $this->orderMock->expects($this->exactly(2))
+        $this->orderMock->expects($this->exactly(3))
             ->method('getBaseShippingAmount')
             ->willReturn(1);
         $this->orderMock->expects($this->once())
@@ -150,7 +150,7 @@ public function testCollectNoBaseShippingAmount()
         $this->orderMock->expects($this->once())
             ->method('getBaseShippingDiscountAmount')
             ->willReturn(1);
-        $this->orderMock->expects($this->exactly(2))
+        $this->orderMock->expects($this->exactly(3))
             ->method('getBaseShippingAmount')
             ->willReturn(1);
         $this->orderMock->expects($this->once())
@@ -269,4 +269,30 @@ public function testCollectZeroShipping()
             );
         $this->assertEquals($this->total, $this->total->collect($this->creditmemoMock));
     }
+
+    /**
+     * @expectedException \Magento\Framework\Exception\LocalizedException
+     * @expectedExceptionMessage You can not refund shipping if there is no shipping amount.
+     */
+    public function testCollectNonZeroShipping()
+    {
+        $this->creditmemoMock->expects($this->once())
+            ->method('setDiscountAmount')
+            ->willReturnSelf();
+        $this->creditmemoMock->expects($this->once())
+            ->method('setBaseDiscountAmount')
+            ->willReturnSelf();
+        $this->creditmemoMock->expects($this->once())
+            ->method('getOrder')
+            ->willReturn($this->orderMock);
+        $this->creditmemoMock->expects($this->once())
+            ->method('getBaseShippingAmount')
+            ->willReturn('10.0000');
+        $this->orderMock->expects($this->never())
+            ->method('getBaseShippingDiscountAmount');
+        $this->orderMock->expects($this->once())
+            ->method('getBaseShippingAmount')
+            ->willReturn( '0.0000');
+        $this->assertEquals($this->total, $this->total->collect($this->creditmemoMock));
+    }
 }