Union type auto reduction/inference on pattern matches.

[Swoorup]

In typescript I can do like the following:

type U = "A" | "B" | "C"
function uBut(x: U): "B" | "C" {
  switch (x) {
    case "A":
      return "B"
    default:
      return x
  }
}

Here, x is deduced to a union of "B" | "C".

With scala 3 union unfortunately you can't do so at the moment. The following fails to compile:

type U = Int | String | Boolean

def uButString(u: U): Int | Boolean =
  u match
    case s: String => 0
    case x => x

This will lead people to instead use asInstanceOf which is bad imo. You can still make it typesafe if you annotate the last type, without warnings like

type U = Int | String | Boolean

def uButString(u: U): Int | Boolean =
  u match
    case s: String => 0
    case x: (Int | Boolean) => x

But it would be nice, if scala was able to auto deduce the type of x to Int | Boolean, leading to more natural code and less usage of asInstanceOf.

If there was a condition on the pattern match, we could retain the current behavior as is.

[Swoorup]

This can be archived, as I created a feature request on dotty

[dwijnand]

lampepfl/dotty-feature-requests#223