Handle single-node child template bodies in cleanup

src/Parser.php

@@ -121,7 +121,7 @@ class Parser
         }
 
         if ($this->parent) {
-            $this->cleanupBodyForChildTemplates($body);
+            $body = $this->cleanupBodyForChildTemplates($body);
         }
 
         $node = new ModuleNode(
@@ -552,8 +552,12 @@ class Parser
         return $test;
     }
 
-    private function cleanupBodyForChildTemplates(Node $body): void
+    private function cleanupBodyForChildTemplates(Node $body): Node
     {
+        if ($body instanceof BlockReferenceNode || ($body instanceof TextNode && $body->isBlank())) {
+            return new EmptyNode();
+        }
+
         foreach ($body as $k => $node) {
             if ($node instanceof BlockReferenceNode) {
                 // as it has a parent, the block reference won't be used
@@ -563,6 +567,8 @@ class Parser
                 $body->removeNode($k);
             }
         }
+
+        return $body;
     }
 
     private function checkPrecedenceDeprecations(ExpressionParserInterface $expressionParser, AbstractExpression $expr)

tests/ParserTest.php

@@ -171,6 +171,17 @@ EOF, 'index')));
         $this->assertSame('set', $body->getNode('4')->getNodeTag());
     }
 
+    public function testCleanupBodyForChildTemplatesWithASingleNodeBody()
+    {
+        $twig = new Environment(new ArrayLoader());
+        $twig->addTokenParser(new ParentSettingTokenParser());
+
+        $node = $twig->parse($twig->tokenize(new Source('{% set_parent %}', 'index')));
+
+        $body = $node->getNode('body')->getNode('0');
+        $this->assertInstanceOf(EmptyNode::class, $body);
+    }
+
     public function testBodyForParentTemplates()
     {
         $twig = new Environment(new ArrayLoader());
@@ -231,3 +242,20 @@ class TestTokenParser extends AbstractTokenParser
         return 'test';
     }
 }
+
+class ParentSettingTokenParser extends AbstractTokenParser
+{
+    public function parse(Token $token): Node
+    {
+        $this->parser->setParent(new ConstantExpression('base', $token->getLine()), false);
+        $this->parser->getStream()->expect(Token::BLOCK_END_TYPE);
+
+        // returns a blank text node so the whole child body is a single removable node
+        return new TextNode('   ', $token->getLine());
+    }
+
+    public function getTag(): string
+    {
+        return 'set_parent';
+    }
+}