chore: add regression test for #13889

Author: hamzaremmal

tests/pos/i13889.scala

@@ -0,0 +1,17 @@
+class DiscordRecord(fields: Map[String, Any]) extends Selectable {
+  def selectDynamic(name: String): Any = fields(name)
+}
+
+type IdentifyScope = DiscordRecord {
+  val username: String
+}
+
+type IncludeIf[Stated <: String, Requirement <: String, IfMatch] = Seq[Stated] match {
+  case Seq[Requirement] => IfMatch
+  case _ => Any
+}
+
+type RawUser[Scopes <: String] = DiscordRecord & IncludeIf[Scopes, "identify", IdentifyScope]
+
+val obj: RawUser["identify"] = ???
+val _ = obj.username