in src/Reflection/ReflectionXHPAttribute.hack [66:81]
public function getValueClass(): string {
$t = $this->getValueType();
invariant(
$t === XHPAttributeType::TYPE_OBJECT,
'Tried to get value class for attribute %s of type %s - needed '.'OBJECT',
$this->getName(),
XHPAttributeType::getNames()[$t],
);
$v = $this->extraType;
invariant(
$v is string,
'Class name for attribute %s is not a string',
$this->getName(),
);
return $v;
}