The class is not final, so we merge it with eventually parent set parameters.

which parent set parameters are you talking about ?

A class not final like RequestContext can be extends and call inside it constructor before the parent construct the setter methods:

class MyCustomContext extends RequestContext {
     public function __construct()
     {
          $this->setParameters(['key' => 'value']);
          parent::__construct();
     }
}

See also new added test. Doing just setParameters($parameters); would override existing parameters of that class.

we could also create a deprecation if (count($this->parameters) > 0) { and simplify this in Symfony 8?

One other question is maybe what is expected behaviour for:

$this->setParameters(['key' => 'value']);
parent::__construct(parameters: ['other' => 'test']);

If not merged we could go for bc layer with:

if (count($parameters) > 0) {
    $this->setParameters($parameters);
} elseif (count($this->parameters) > 0) {
    trigger_deprecation('symfony/routing', '7.3', 'Set parameters directly via the own constructor argument');
}

which in 8.0 gets:

$this->setParameters($parameters);

not sure what this test case is about

its about why the if is needed for BC reasons.

This lines. As the require for the bc case in this tests, other tests wouldn't catch that.

+        if ($parameters) {
            $this->parameters = $parameters;
+        }

First I wanted to trigger deprecation so we could just remove it with 8.0.

@nicolas-grekas or should we think of that nobody did call setParameters before calling __construct then we can remove the if and this test completely.