Short version: If X inherits from Y, then X should pass all of Y’s black box tests. I first encountered this idea at SPLASH 2021. The longer explanation A bit of background In A Behavioral Notion of Subtyping Liskov originally defined subtyping in inherited objects as follows: Subtype Requirement: Let P(x) be a property provable about objects x of type T. Then P(y) should be true for objects y of type S where S is a subtype of T.
