What are covariance and contravariance?

Question presented to candidate: "If a Dog is assignable to an Animal, is a Dog[] assignable to an Animal[]? Is a (d: Dog) => void assignable to an (a: Animal) => void? Explain covariance and contravariance, and where TypeScript is deliberately unsound."

What a strong answer should cover:

• Variance describes how a subtype relation between Dog and Animal carries over to a type built from them, such as Box<Dog> and Box<Animal>: same direction (covariant), reversed (contravariant), neither (invariant), or both (bivariant)
• Outputs (return types, read-only properties) are covariant; inputs (parameters) are contravariant, because a function that accepts any Animal can safely be used where only Dogs will be passed
• strictFunctionTypes (TypeScript 2.6, part of --strict) checks function-type parameters contravariantly, but parameters declared with method syntax stay bivariant on purpose
• Mutable arrays are treated as covariant, which is unsound: you can push a Cat into a Dog[] through an Animal[] alias; readonly arrays are covariant and safe
• A generic type that both reads and writes T is invariant; TypeScript measures variance from the structure, and in / out annotations (TypeScript 4.7) declare and check it

Clarifying questions expected:

• Is strictFunctionTypes (or strict) on in this codebase?
• Are the callbacks declared with method syntax or as function-typed properties?

Code / implementation expected: Yes — assign a Dog-only callback where an Animal callback is expected, show the compile error, and show the array hole at run time.