Scala WTF?

WTF.scala

case class X[A](a: A)

object X {
  // succeeds
  def q: X[String] = X("abc")

  // succeeds
  def r: X[Unit] = X("abc")

  // fails
  // def s: X[Unit] = q

  // fails
  // def t: X[Unit] = X[String]("abc")

  // succeeds
  def s: X[Unit] = X[Unit]("abc")
}

It's taking you to mean X.apply[Unit]{ "abc"; () }

Scala Language Spec §6.26.1 (Value Conversions): "If e has some value type and the expected type is Unit, e is converted to the expected type by embedding it in the term { e; () }"

Rúnar beat me to it :P

scala29 -Ywarn-value-discard -e 'case class X[A](a: A); def r: X[Unit] = X("abc")'
/var/folders/xq/4pqxykpn5n56mngmch4p5xgw0000gn/T/scalacmd5125780711720939650.scala:1: warning: discarded non-Unit value
case class X[A](a: A); def r: X[Unit] = X("abc")
                                          ^