I think variance can be simulated by typing the private field as an existing covariant/contravariant/invariant type

  class CovariantFoo<T> {
    private phantom!: T
  }

  class ContravariantFoo<T> {
    private phantom!: (_: T) => void
  }

  class InvariantFoo<T> {
    private phantom!: (_: T) => T
  }

(inspired by Rust [0])

[0] https://doc.rust-lang.org/nomicon/phantom-data.html