scala3-UnionTypes6.scala

val f123a: (T1 | T2 | T3) => String = _ match
  case t1: T1 => "T1"
  case t2: T2 => "T2"
  case t3: T3 => "T3"

val f123b: (T1 => String) & (T2 => String) & (T3 => String) = f123a

val t123s: Seq[T1 | T2 | T3] = Seq(new T1 {}, new T2 {}, new T3 {})
t123s.map(f123a)
t123s.map(f123b)

// Note that we can pass f123a or f123b for a function T1 => String. It works similarly for T2 and T3.
def stringizeT1(t1: T1)(f: T1 => String): String = f(t1)
stringizeT1(new T1 {})(f123a)
stringizeT1(new T1 {})(f123b)