Evidence that A <: B, taking |-types into account.
Operations on union types.
Upcast A to B1 | B2.
Upcast A to B1 | B2.
This needs evidence that A <: B1 | B2.
Upcast F[A] to F[B].
Upcast F[A] to F[B].
This needs evidence that F[A] <: F[B].
Provides an Option-like API to js.UndefOr.