I propose to add the following constraints for validating Traversable instances and arrays:

• Each: Validates that each entry satisfies some constraint (replaces All, which should be deprecated).
• Some: Validates that at least one entry satisfies some constraint. The lower limit (default 1) and upper limit (default null, i.e. unbounded) can be configured. Alternatively, a precise number of matches can be given.
• None: Validates that 0 entries satisfy some constraint (alias for Some(exactly = 0, ...))
• Unique: Validates that the collection does not contain duplicates.

Example:

/**
 * @Each(@GreaterThan(6))
 */
private $numbers.

/**
 * @Some(@GreaterThan(6))
 */
private $numbers.

/**
 * @Some(@GreaterThan(6), min = 2, max = 5)
 */
private $numbers.

/**
 * @Some(@GreaterThan(6), exactly = 3)
 */
private $numbers.

/**
 * @None(@GreaterThan(6))
 */
private $numbers.

/**
 * @Unique
 */
private $numbers.

Each constraint should expect either a single constraint or an array of constraints to be given. That means you can also do:

/**
 * @Each({
 *     @NotNull,
 *     @GreaterThan(6)
 * })
 */
private $numbers.

"min" or "max" and "exactly" must not be given at the same time:

// ConstraintDefinitionException
new Some(array(
    'constraints' => array(...),
    'min' => 2,
    'exactly' => 2,
));

Groups on inner constraints should be supported (ref #4453).

• If a collection constraint has explicit groups given, it should call addImplicitGroupName() on the nested constraints with each of these groups (except "Default").

  new Each(array(
      'groups' => array('Default', 'Strict'),
      'constraints' => new GreaterThan(6), // implicitly group "Default" and "Strict"
  ));

• If a collection constraint has no explicit group given, its groups should be the merged result of all nested groups.

  new Each(array(
      'constraints' => array(
          new GreaterThan(array(
              'value' => 6,
              'groups' => array('Default', 'Strict'),
          )),
          new NotNull(array('groups' => 'Filled')),
      ),
      // implicitly group "Default", "Strict" and "Filled"
  ));

• If both the collection constraint and a nested constraint have explicit groups given, the groups of the nested constraint need to be a subset of the groups of the outer constraint.

  // ok
  new Each(array(
      'groups' => array('Default', 'Strict'),
      'constraints' => new GreaterThan(array(
          'value' => 6,
          'groups' => array('Default', 'Strict'),
      )),
  ));
  
  // ok
  new Each(array(
      'groups' => array('Default', 'Strict'),
      'constraints' => new GreaterThan(array(
          'value' => 6,
          'groups' => 'Strict',
      )),
  ));
  
  // ConstraintDefinitionException
  new Each(array(
      'groups' => array('Default', 'Strict'),
      'constraints' => new GreaterThan(array(
          'value' => 6,
          'groups' => array('Strict', 'Sales'),
      )),
  ));

• Calls to addImplicitGroupName() on the collection constraint should be propagated to nested constraints.