Contributed by wuchen90
in #42593.

The Symfony Validator component includes some advanced constraints such as
Callback (to implement custom validation rules), Sequentially (to apply
a set of rules in order and interrupt them at any point), Compound (to create
a set of reusable constraints), etc.
In Symfony 6.2 we're adding another advanced constraint to that list: When,
which allows to implement conditional validations.
Consider a Discount class with two properties:

// src/Model/Discount.php
namespace App\Model;

class Discount
{
private ?string $type;
private ?int $value;

// ...
}

To validate the object contents, you need to apply these rules:

• If type is percent, then value must be less than or equal 100;
• If type is absolute, then value can be any value;
• In all cases, the value must be greater than 0.

The new When constraint defines two main options called expression and
constraints. These constraints are only enforced when the result of evaluating
the expression is true. You can use it as follows to validate that the
value is less than 100 only if the discount type is percent:

use Symfony\Component\Validator\Constraints as Assert;
// ...

class Discount
{
#[Assert\GreaterThan(0)]
#[Assert\When(
expression: 'this.type == "percent"',
constraints: [
new Assert\LessThanOrEqual(100, message: 'The value should be between 1 and 100!')
],
)]
private ?int $value;

// ...
}

The condition passed to the expression option must use the Symfony
ExpressionLanguage syntax. Inside the expression you can use the this
variable to refer to the object being validated and value to refer to the
property being valuated (this is only available if you apply the When
constraint to properties).
Finally, you can combine When with other advanced constraints such as Callback
to define complex conditional validations:

use Symfony\Component\Validator\Constraints as Assert;
use Symfony\Component\Validator\Context\ExecutionContextInterface;

class Discount
{
#[Assert\When(
expression: 'value == "percent"',
constraints: [new Assert\Callback('doComplexValidation')],
)]
private ?string $type;

// ...
}

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