Mark implicit macro argument default values as such with an attribute in AST

src/ExpressionParser.php

@@ -591,7 +591,7 @@ class ExpressionParser
      * Parses arguments.
      *
      * @param bool $namedArguments Whether to allow named arguments or not
-     * @param bool $definition     Whether we are parsing arguments for a function definition
+     * @param bool $definition     Whether we are parsing arguments for a function (or macro) definition
      *
      * @return Node
      *
@@ -642,6 +642,7 @@ class ExpressionParser
                 if (null === $name) {
                     $name = $value->getAttribute('name');
                     $value = new ConstantExpression(null, $this->parser->getCurrentToken()->getLine());
+                    $value->setAttribute('is_implicit', true);
                 }
                 $args[$name] = $value;
             } else {

tests/ParserTest.php

@@ -14,6 +14,8 @@ namespace Twig\Tests;
 use PHPUnit\Framework\TestCase;
 use Twig\Environment;
 use Twig\Error\SyntaxError;
+use Twig\Lexer;
+use Twig\Loader\ArrayLoader;
 use Twig\Loader\LoaderInterface;
 use Twig\Node\Node;
 use Twig\Node\SetNode;
@@ -175,6 +177,27 @@ EOF
         $this->addToAssertionCount(1);
     }
 
+    public function testImplicitMacroArgumentDefaultValues()
+    {
+        $template = '{% macro marco (po, lo = true) %}{% endmacro %}';
+        $lexer = new Lexer(new Environment(new ArrayLoader()));
+        $stream = $lexer->tokenize(new Source($template, 'index'));
+
+        $argumentNodes = $this->getParser()
+            ->parse($stream)
+            ->getNode('macros')
+            ->getNode('marco')
+            ->getNode('arguments')
+        ;
+
+        $this->assertTrue($argumentNodes->getNode('po')->hasAttribute('is_implicit'));
+        $this->assertTrue($argumentNodes->getNode('po')->getAttribute('is_implicit'));
+        $this->assertNull($argumentNodes->getNode('po')->getAttribute('value'));
+
+        $this->assertFalse($argumentNodes->getNode('lo')->hasAttribute('is_implicit'));
+        $this->assertSame(true, $argumentNodes->getNode('lo')->getAttribute('value'));
+    }
+
     protected function getParser()
     {
         $parser = new Parser(new Environment($this->createMock(LoaderInterface::class)));