twigphp/Twig · error · RuntimeError

You cannot use the Twig function "constant" to access

Error message

You cannot use the Twig function "constant" to access "%s". You could provide an object and call constant("class", $object) or use the class name directly as a string.

What it means

CoreExtension::constant() backs Twig's `constant()` function. When the requested constant is not defined and its name ends with `::class` (case-insensitive), Twig throws this specific RuntimeError because `Foo::class` is a compile-time PHP construct, not a runtime constant — `defined('Foo::class')` is always false. The error tells the developer to use the object form `constant('class', $object)` or pass the FQCN string directly.

Solutions

Pass the object you have to the function: `constant('class', object)`
If you only have the class name string, use it directly: `{{ 'App\\Entity\\User' }}` instead of `constant('App\\Entity\\User::class')`
Use the `class` helper / `enum` function if available in your Twig version for class/enum resolution
Wrap with `constant(..., checkDefined=true)` semantics (via `defined`) if the constant may legitimately be absent

Example fix

// before (Twig template)
{{ constant('App\\Entity\\User::class') }}
// after
{{ constant('class', user) }}
{{ constant('SOME_CONST', 'App\\Entity\\User'|enum) }}

Defensive patterns

Strategy: type-guard

Validate before calling

// PHP, before rendering a template that uses constant('X::class')
if (is_object($value)) {
    // use constant('class', $value) in the template
} else {
    // pass the FQCN string directly to the template
}

Type guard

function canUseConstant(string $name, ?object $object = null): bool
{
    if (null !== $object) {
        return 'class' === $name || defined($object::class.'::'.$name);
    }
    return defined($name) && '::class' !== strtolower(substr($name, -7));
}

Try / catch

try {
    $value = $twigExtensionConstant($name, $object);
} catch (\Twig\Error\RuntimeError $e) {
    // ::class on a string class name: use the string itself or constant('class', $object)
    $value = null;
}

Prevention

Never write `Foo::class` inside Twig constant(); pass objects or FQCN strings directly
Prefer the `constant('class', $object)` form when an instance is in scope
Search templates for the `::class` suffix as a lint rule
Upgrade templates when migrating Twig versions that tightened constant() handling

When it happens

Trigger: Calling `{{ constant('Some\\Class::class') }}` in a template with only a string class name, or `constant('::class')` style usage, where the class exists but the `::class` pseudo-constant cannot be resolved via defined()/constant(). Not thrown when $checkDefined is true (returns false instead).

Common situations: Templates migrated from PHP-land habits where `Foo::class` works in code but not through constant(); trying to get a class name inside Twig from a string; dynamic class-name lookup in older templates after Twig tightened the check.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of twigphp/Twig@a414c3a491 (2026-09-13). Data as JSON: /api/errors/8961e74d79e15f26. Report an issue: GitHub.

Appendix: source

Thrown at src/Extension/CoreExtension.php:1692

     * @internal
     */
    public static function constant($constant, $object = null, bool $checkDefined = false)
    {
        if (null !== $object) {
            if ('class' === $constant) {
                return $checkDefined ? true : $object::class;
            }

            $constant = $object::class.'::'.$constant;
        }

        if (!\defined($constant)) {
            if ($checkDefined) {
                return false;
            }

            if ('::class' === strtolower(substr($constant, -7))) {
                throw new RuntimeError(\sprintf('You cannot use the Twig function "constant" to access "%s". You could provide an object and call constant("class", $object) or use the class name directly as a string.', $constant));
            }

            throw new RuntimeError(\sprintf('Constant "%s" is undefined.', $constant));
        }

        return $checkDefined ? true : \constant($constant);
    }

    /**
     * Batches item.
     *
     * @param array $items An array of items
     * @param int   $size  The size of the batch
     * @param mixed $fill  A value used to fill missing items
     *
     * @internal
     */
    public static function batch($items, $size, $fill = null, $preserveKeys = true): array

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