Learning about the Liskov principle, I do understand that preconditions can be weakened and postconditions can be stronger in subtypes. However, I do not understand how invariants can be stronger in the subtype? If the base class says e.g. the Speed must be lower than 100 al the time, and then a subtype says Speed < 50, replacing the base with the subtype can cause some issues, or not? I mean, if somewhere the code works with the base class and works the speed of 90, introducing the subtype means it will be invalid.
Code and some text can be found in the slides 15=16:
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