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function method(value: Invariant<City>) {...}
method(new Noun());         // error...
method(new City());         // okay
method(new SanFrancisco()); // error...

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function method(value: Covariant<City>) {...}
method(new Noun());         // error...
method(new City());         // okay
method(new SanFrancisco()); // okay

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function method(value: Contravariant<City>) {...}
method(new Noun());         // okay
method(new City());         // okay
method(new SanFrancisco()); // error...

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function method(value: Bivariant<City>) {...}
method(new Noun());         // okay
method(new City());         // okay
method(new SanFrancisco()); // okay

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// Equally specific inputs and outputs — Good
class SubClass extends BaseClass {
  method(value: City): City { ... }
}

// More specific outputs — Good
class SubClass extends BaseClass {
  method(value: City): SanFrancisco { ... }
}

// Less specific inputs — Good
class SubClass extends BaseClass {
  method(value: Noun): City { ... }
}

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// Less specific outputs — Bad
class SubClass extends BaseClass {
  method(value: City): Noun { ... } // ERROR!!
}

// More specific inputs — Bad
class SubClass extends BaseClass {
  method(value: SanFrancisco): City { ... } // ERROR!!
}