Currently, the validator will traverse objects that implement \Traversable if

• the object was reached via a Valid constraint with traverse set to true
• the parameter $traverse was set to true when calling Validator::validate()

This has two negative consequences:

• it's impossible to specify only once that a specific class should never/always be traversed
• it's impossible to specify whether an object should be traversed when Validator::validate() is not called manually, but through another system (e.g. the Form component - see [Form][Bug] Embedded form validation bubbling #8557)

A solution would be to add configuration parameters for specifying the traversal policy on a class level, for example:

// not traversed by default
class Foo implements \IteratorAggregate
{
    // ...
}

// traversed
/** @Assert\Traverse */
class Bar implements \IteratorAggregate
{
}

// traversed recursively
/** @Assert\Traverse(deep=true) */
class Baz implements \IteratorAggregate
{
}