Skip the string cast in PrintNode when the expression is already a string and add tests

CHANGELOG

@@ -1,6 +1,7 @@
 # 3.27.2 (2026-XX-XX)
 
  * Make the `IntegrationTestCase` and `NodeTestCase` test helpers compatible with PHPUnit 11
+ * Cast printed expressions to string so values that cannot be converted to a string (arrays, non-`Stringable` objects, ...) report a usable stack trace at the print location
  * Skip the sandbox `__toString` check on arguments whose PHP parameter type cannot implicitly coerce to string
 
 # 3.27.1 (2026-05-30)

src/Node/PrintNode.php

@@ -15,6 +15,8 @@ namespace Twig\Node;
 use Twig\Attribute\YieldReady;
 use Twig\Compiler;
 use Twig\Node\Expression\AbstractExpression;
+use Twig\Node\Expression\ConstantExpression;
+use Twig\Node\Expression\ReturnStringInterface;
 
 /**
  * Represents a node that outputs an expression.
@@ -34,9 +36,18 @@ class PrintNode extends Node implements NodeOutputInterface, CoercesChildrenToSt
         /** @var AbstractExpression */
         $expr = $this->getNode('expr');
 
+        $compiler->addDebugInfo($this);
+
+        if ($expr->isGenerator()) {
+            $compiler->write('yield from ');
+        } else {
+            $compiler->write('yield ');
+            if (!$this->isString($expr)) {
+                $compiler->raw('(string) ');
+            }
+        }
+
         $compiler
-            ->addDebugInfo($this)
-            ->write($expr->isGenerator() ? 'yield from ' : 'yield (string) ')
             ->subcompile($expr)
             ->raw(";\n")
         ;
@@ -46,4 +57,13 @@ class PrintNode extends Node implements NodeOutputInterface, CoercesChildrenToSt
     {
         return ['expr'];
     }
+
+    private function isString(AbstractExpression $expr): bool
+    {
+        if ($expr instanceof ReturnStringInterface) {
+            return true;
+        }
+
+        return $expr instanceof ConstantExpression && !$expr->isDefinedTestEnabled() && \is_string($expr->getAttribute('value'));
+    }
 }

tests/Node/ForTest.php

@@ -73,7 +73,7 @@ class ForTest extends NodeTestCase
 \$context['_parent'] = \$context;
 \$context['_seq'] = CoreExtension::ensureTraversable($itemsGetter);
 foreach (\$context['_seq'] as \$context["key"] => \$context["item"]) {
-    yield $fooGetter;
+    yield (string) $fooGetter;
 }
 \$_parent = \$context['_parent'];
 unset(\$context['_seq'], \$context['key'], \$context['item'], \$context['_parent']);
@@ -107,7 +107,7 @@ if (is_array(\$context['_seq']) || (is_object(\$context['_seq']) && \$context['_
     \$context['loop']['last'] = 1 === \$length;
 }
 foreach (\$context['_seq'] as \$context["k"] => \$context["v"]) {
-    yield $fooGetter;
+    yield (string) $fooGetter;
     ++\$context['loop']['index0'];
     ++\$context['loop']['index'];
     \$context['loop']['first'] = false;
@@ -149,7 +149,7 @@ if (is_array(\$context['_seq']) || (is_object(\$context['_seq']) && \$context['_
     \$context['loop']['last'] = 1 === \$length;
 }
 foreach (\$context['_seq'] as \$context["k"] => \$context["v"]) {
-    yield $fooGetter;
+    yield (string) $fooGetter;
     ++\$context['loop']['index0'];
     ++\$context['loop']['index'];
     \$context['loop']['first'] = false;
@@ -192,7 +192,7 @@ if (is_array(\$context['_seq']) || (is_object(\$context['_seq']) && \$context['_
     \$context['loop']['last'] = 1 === \$length;
 }
 foreach (\$context['_seq'] as \$context["k"] => \$context["v"]) {
-    yield $fooGetter;
+    yield (string) $fooGetter;
     \$context['_iterated'] = true;
     ++\$context['loop']['index0'];
     ++\$context['loop']['index'];
@@ -206,7 +206,7 @@ foreach (\$context['_seq'] as \$context["k"] => \$context["v"]) {
 // line 5
 if (!\$context['_iterated']) {
     // line 6
-    yield $fooGetter;
+    yield (string) $fooGetter;
 }
 \$_parent = \$context['_parent'];
 unset(\$context['_seq'], \$context['k'], \$context['v'], \$context['_parent'], \$context['_iterated'], \$context['loop']);

tests/Node/IfTest.php

@@ -63,7 +63,7 @@ class IfTest extends NodeTestCase
         $tests[] = [$node, <<<EOF
 // line 1
 if (true) {
-    yield $fooGetter;
+    yield (string) $fooGetter;
 }
 EOF
         ];
@@ -80,9 +80,9 @@ EOF
         $tests[] = [$node, <<<EOF
 // line 1
 if (true) {
-    yield $fooGetter;
+    yield (string) $fooGetter;
 } elseif (false) {
-    yield $barGetter;
+    yield (string) $barGetter;
 }
 EOF
         ];
@@ -97,9 +97,9 @@ EOF
         $tests[] = [$node, <<<EOF
 // line 1
 if (true) {
-    yield $fooGetter;
+    yield (string) $fooGetter;
 } else {
-    yield $barGetter;
+    yield (string) $barGetter;
 }
 EOF
         ];

tests/Node/PrintTest.php

@@ -20,6 +20,7 @@ namespace Twig\Tests\Node;
  * file that was distributed with this source code.
  */
 
+use Twig\Node\Expression\Binary\ConcatBinary;
 use Twig\Node\Expression\ConstantExpression;
 use Twig\Node\Expression\GetAttrExpression;
 use Twig\Node\Expression\Variable\ContextVariable;
@@ -40,8 +41,20 @@ class PrintTest extends NodeTestCase
     public static function provideTests(): iterable
     {
         $tests = [];
+
+        // a string literal is known to be a string, so the cast is skipped
         $tests[] = [new PrintNode(new ConstantExpression('foo', 1), 1), "// line 1\nyield \"foo\";"];
 
+        // a non-string expression is cast to make the conversion happen here for a useful stack trace
+        $tests[] = [new PrintNode(new ContextVariable('foo', 1), 1), "// line 1\nyield (string) (\$context[\"foo\"] ?? null);"];
+
+        // a non-string constant is cast as well
+        $tests[] = [new PrintNode(new ConstantExpression(42, 1), 1), "// line 1\nyield (string) 42;"];
+
+        // a concatenation always returns a string, so the cast is skipped
+        $concat = new ConcatBinary(new ContextVariable('foo', 1), new ContextVariable('bar', 1), 1);
+        $tests[] = [new PrintNode($concat, 1), "// line 1\nyield ((\$context[\"foo\"] ?? null) . (\$context[\"bar\"] ?? null));"];
+
         $expr = new ContextVariable('foo', 1);
         $attr = new ConstantExpression('bar', 1);
         $node = new GetAttrExpression($expr, $attr, null, Template::METHOD_CALL, 1);

tests/TemplateTest.php

@@ -376,6 +376,35 @@ class TemplateTest extends TestCase
         $this->assertNull(CoreExtension::getAttribute($twig, $template->getSourceContext(), $object, 'foo'));
     }
 
+    /**
+     * @dataProvider provideNonStringPrintValues
+     */
+    public function testPrintingANonStringReportsTheErrorAtThePrintLocation($value, string $expectedMessage)
+    {
+        $twig = new Environment(new ArrayLoader(['index' => "foo\n{{ value }}\nbar"]));
+
+        set_error_handler(static function (int $type, string $msg, string $file, int $line): bool {
+            throw new \ErrorException($msg, 0, $type, $file, $line);
+        }, \E_WARNING);
+
+        try {
+            $twig->render('index', ['value' => $value]);
+            $this->fail('Printing a non-string value should fail.');
+        } catch (RuntimeError $e) {
+            $this->assertSame('index', $e->getSourceContext()->getName());
+            $this->assertSame(2, $e->getTemplateLine());
+            $this->assertSame($expectedMessage, $e->getPrevious()->getMessage());
+        } finally {
+            restore_error_handler();
+        }
+    }
+
+    public static function provideNonStringPrintValues(): iterable
+    {
+        yield 'array' => [['a', 'b'], 'Array to string conversion'];
+        yield 'non-Stringable object' => [new \stdClass(), 'Object of class stdClass could not be converted to string'];
+    }
+
     public static function getGetAttributeTests()
     {
         $array = [