Fix #3946: Add Unit lifter in quoted.Liftable

compiler/src/dotty/tools/dotc/quoted/Runners.scala

@@ -3,11 +3,11 @@ package dotty.tools.dotc.quoted
 import dotty.tools.dotc.ast.Trees._
 import dotty.tools.dotc.ast.tpd
 import dotty.tools.dotc.core.Constants._
-import dotty.tools.dotc.core.Contexts._
 import dotty.tools.dotc.printing.RefinedPrinter
 
 import scala.quoted.Expr
 import scala.quoted.Liftable.ConstantExpr
+import scala.runtime.BoxedUnit
 import scala.runtime.quoted._
 
 /** Default runners for quoted expressions */
@@ -23,7 +23,8 @@ object Runners {
         implicit val ctx = new QuoteDriver().initCtx
         ctx.settings.color.update("never")
         val printer = new RefinedPrinter(ctx)
-        printer.toText(Literal(Constant(expr.value))).mkString(Int.MaxValue, false)
+        if (expr.value == BoxedUnit.UNIT) "()"
+        else printer.toText(Literal(Constant(expr.value))).mkString(Int.MaxValue, false)
       case _ => new QuoteDriver().show(expr)
     }
 

library/src/scala/quoted/Liftable.scala

@@ -22,6 +22,7 @@ object Liftable {
     override def toString: String = s"Expr($value)"
   }
 
+  implicit def UnitIsLiftable: Liftable[Unit] = (x: Unit) => new ConstantExpr(x)
   implicit def BooleanIsLiftable: Liftable[Boolean] = (x: Boolean) => new ConstantExpr(x)
   implicit def ByteLiftable: Liftable[Byte] = (x: Byte) => new ConstantExpr(x)
   implicit def CharIsLiftable: Liftable[Char] = (x: Char) => new ConstantExpr(x)

tests/pos/quote-liftable.scala

@@ -33,6 +33,7 @@ object Test {
   (1.0f: Expr[Float])
   (1.0: Expr[Double])
   ("abc": Expr[String])
+  ((): Expr[Unit])
 
   val xs: Expr[List[Int]] = 1 :: 2 :: 3 :: Nil
 }

tests/run-with-compiler/i3946.check

@@ -0,0 +1,2 @@
+()
+()

tests/run-with-compiler/i3946.scala

@@ -0,0 +1,9 @@
+import dotty.tools.dotc.quoted.Runners._
+import scala.quoted._
+object Test {
+  def main(args: Array[String]): Unit = {
+    val u: Expr[Unit] = ()
+    println(u.show)
+    println(u.run)
+  }
+}