No implicit Show for union of singleton types in REPL

[senia-psm]

scala> trait TmpT { type A; type B; type AorB = A | B ; def fromA(a: A): AorB = a; def fromB(b: B): AorB = b } 
// defined trait TmpT
scala> object TmpO extends TmpT{ type A = 7; type B = 8 } 
// defined object TmpO
scala> val x: TmpO.AorB = TmpO.fromA(7) 
1 |val x: TmpO.AorB = TmpO.fromA(7)
  |^
  |no implicit argument of type dotty.Show[TmpO.AorB] was found for parameter ev of method show in class ShowValue.
  |I found:
  |
  |    dotty.Show.defaultShow[Nothing]
  |
  |But method defaultShow in trait LowPrioShow does not match type dotty.Show[TmpO.AorB].

Note that one can fix it using this line:

implicit val showAorB: dotty.Show[TmpO.AorB] = dotty.Show.defaultShow[TmpO.AorB]

[Blaisorblade]

Actually, @smarter at https://gitter.im/lampepfl/dotty?at=5a892b7035dd17022ebc5968 suggests the area:repl label, but the compile error can be reproduced outside the REPL (see below).

So there are two issues:

1. union of singleton types are not supported currently

2. But the repl shouldn't give confusing errors like this

For 2, I guess it should explain why it's searching for a Show instance, since that's not something the user explicitly asked for. Probably prepend the error with something like "error while trying to display result".

Non-REPL variant:

trait TmpT { type A; type B; type AorB = A | B ; def fromA(a: A): AorB = a; def fromB(b: B): AorB = b }
object TmpO extends TmpT{ type A = 7; type B = 8 }
object Tester {
  val x: TmpO.AorB = TmpO.fromA(7)
  //implicit val showAorB: dotty.Show[TmpO.AorB] = dotty.Show.defaultShow[TmpO.AorB]
  implicitly[dotty.Show[TmpO.AorB]]
}