Status: Needs Review

Thank you @gharlan.

This makes compatibility for older versions much harder as you still need to call root() in those cases, while passing the name to the constructor was was already possible.

$treeBuilder = new TreeBuilder('rollerworks_password_strength');
$rootNode = $treeBuilder->root('rollerworks_password_strength'); // BC for Symfony < 4.2

What about checking if a name was already set in the constructor, and it equals the passed name.
In this case, return $this->root and do nothing, otherwise throw an deprecation instead.

To ensure forward compatibility you still need to check if the root() method exists. But least you don't get any deprecation warnings.

$treeBuilder = new TreeBuilder('rollerworks_password_strength');

if (method_exists($treeBuilder, 'root')) {
    $rootNode = $treeBuilder->root('rollerworks_password_strength'); // BC for Symfony < 4.2
} else {
    $rootNode = $treeBuilder->getRootNode();
}

As far as I know many libs already use method_exists.

Ah I completely missed #27476 😛

#27476 (comment) gives a good suggestion

$treeBuilder = new TreeBuilder("my_node");
$rootNode = method_exists($treeBuilder, "getRootNode")
    ? $treeBuilder->getRootNode()
    : $treeBuilder->root("my_node");

👍