If A1 <: B and A2 <: B, then A1 | A2 <: B.
A <: A.
Int <: Double, because that's true in Scala.js.
Int <: Double, because that's true in Scala.js.
If A <: B1, then A <: B1 | B2.
If A <: B1, then A <: B1 | B2.
If A <: B2, then A <: B1 | B2.
If A <: B2, then A <: B1 | B2.
If A <: B, then A <: js.UndefOr[B].
If A <: B, then A <: js.UndefOr[B].