The code contains following hierarchy of classes:
---
title: Classes hierarchy
---
classDiagram
AbstractGadget <|-- DoubleGadget
AbstractGadget <|-- MultiplierGadget
AbstractGadget: +apply(float $multiplier, int $value)
MultiplierGadget: +apply(float $multiplier, int $value)
DoubleGadget: +apply(float $multiplier = 2.0, int $value)
One of the classes defines not last parameter of inherited method to be optional, which is deprecated in 8.0. In this case rector fixes the child class method, but neither the parent method definition, nor all sibling classes to follow that change.