hanle Unit as a decomposable type with a single value

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -443,6 +443,8 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
           Typ(ConstantType(Constant(true)), true),
           Typ(ConstantType(Constant(false)), true)
         )
+      case tp if tp.isRef(defn.UnitClass) =>
+        Typ(ConstantType(Constant(())), true) :: Nil
       case tp if tp.classSymbol.is(Enum) =>
         children.map(sym => Typ(sym.termRef, true))
       case tp =>
@@ -708,6 +710,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
         canDecompose(and.tp1) || canDecompose(and.tp2)
       }) ||
       tp.isRef(defn.BooleanClass) ||
+      tp.isRef(defn.UnitClass) ||
       tp.classSymbol.is(allOf(Enum, Sealed))  // Enum value doesn't have Sealed flag
 
     debug.println(s"decomposable: ${tp.show} = $res")

tests/patmat/i4227b.scala

@@ -0,0 +1,16 @@
+sealed abstract class Maybe[A]
+final case class Just[A](a: A) extends Maybe[A]
+class Empty[A] extends Maybe[A]
+object Empty {
+  def apply[A](): Maybe[A] = new Empty[A]
+  def unapply[A](e: Empty[A]): Some[Unit] = Some(())
+}
+
+object Test {
+  val a: Maybe[Int] = Just(2)
+  def main(args: Array[String]): Unit = a match {
+    case Just(_) =>
+    // case Empty(_) => // ok
+    case Empty(()) => // match may not be exhaustive. It would fail on: Empty(_)
+  }
+}