Scala 3: Matching on member of a union types

inline.scala

inline def f2[A, B](x: A | B): String =
  x match
    case a: A => "a"
    case b: B => "b"
    
// doesn't work with subtyping

trait T[A] {
  def foo(a: A): String
}

class Union[A, B](ta: T[A], tb: T[B]) extends T[A | B]{
  inline def foo(a: A | B): String = a match
    case a: A => ta.foo(a)
    case b: B => tb.foo(b)
}

def t[A] = new T[A]{ def foo(a: A): String = a.toString }

val union = new Union(t[String], t[Int])

println(union.foo("foo"))
println(union.foo(1))

typable.scala

import scala.reflect.Typeable

def f[A: Typeable, B: Typeable](x: A | B): String =
  x match
    case a: A => "a"
    case b: B => "b"
    
// works with subtyping

trait T[A] {
  def foo(a: A): String
}

class Union[A: Typeable, B: Typeable](ta: T[A], tb: T[B]) extends T[A | B]{
  def foo(a: A | B): String = a match
    case a: A => ta.foo(a)
    case b: B => tb.foo(b)
}

def t[A] = new T[A]{ def foo(a: A): String = a.toString }

val union = new Union(t[String], t[Int])

println(union.foo("foo"))
println(union.foo(1))