made Template::getSourceContext() abstract

src/Template.php

@@ -66,10 +66,7 @@ abstract class Template
      *
      * @return Source
      */
-    public function getSourceContext()
-    {
-        return new Source('', $this->getTemplateName());
-    }
+    abstract public function getSourceContext();
 
     /**
      * Returns the parent template.

test/Twig/Tests/TemplateTest.php

@@ -16,6 +16,7 @@ use Twig\Loader\ArrayLoader;
 use Twig\Loader\LoaderInterface;
 use Twig\Sandbox\SecurityError;
 use Twig\Sandbox\SecurityPolicy;
+use Twig\Source;
 use Twig\Template;
 
 class Twig_Tests_TemplateTest extends \PHPUnit\Framework\TestCase
@@ -440,6 +441,11 @@ class Twig_TemplateTest extends Template
         return [];
     }
 
+    public function getSourceContext()
+    {
+        return new Source('', $this->getTemplateName());
+    }
+
     protected function doGetParent(array $context)
     {
         return false;