diff --git a/compiler/src/dotty/tools/dotc/ast/MainProxies.scala b/compiler/src/dotty/tools/dotc/ast/MainProxies.scala
index 5e969c0c38c9..1133d015f69f 100644
--- a/compiler/src/dotty/tools/dotc/ast/MainProxies.scala
+++ b/compiler/src/dotty/tools/dotc/ast/MainProxies.scala
@@ -242,8 +242,10 @@ object MainProxies {
         val param = paramName.toString
         val paramType0 = if formal.isRepeatedParam then formal.argTypes.head.dealias else formal.dealias
         val paramType = paramType0.dealias
-
-        val paramTypeStr = formal.dealias.typeSymbol.owner.showFullName + "." + paramType.show
+        val paramTypeOwner = paramType.typeSymbol.owner
+        val paramTypeStr =
+          if paramTypeOwner == defn.EmptyPackageClass then paramType.show
+          else paramTypeOwner.showFullName + "." + paramType.show
         val hasDefault = defaultValueSymbols.contains(idx)
         val isRepeated = formal.isRepeatedParam
         val paramDoc = documentation.argDocs.getOrElse(param, "")
diff --git a/tests/run/main-calculator-example.check b/tests/run/main-calculator-example.check
new file mode 100644
index 000000000000..07946c911e9f
--- /dev/null
+++ b/tests/run/main-calculator-example.check
@@ -0,0 +1,8 @@
+Expression: Plus(Number(1),Number(2))
+Calculated: 3
+executing sum2 with inputs: 2 3
+Expression: Plus(Number(2),Number(3))
+Calculated: 5
+executing sumAll with inputs: 1 2 3
+Expression: Plus(Plus(Number(1),Number(2)),Number(3))
+Calculated: 6
diff --git a/tests/run/main-calculator-example.scala b/tests/run/main-calculator-example.scala
new file mode 100644
index 000000000000..813f63c956a8
--- /dev/null
+++ b/tests/run/main-calculator-example.scala
@@ -0,0 +1,64 @@
+sealed trait Expression:
+  def eval(): Int
+case class Number(n: Int) extends Expression:
+  def eval(): Int = n
+case class Plus(e1: Expression, e2: Expression) extends Expression:
+  def eval(): Int = e1.eval() + e2.eval()
+
+////
+
+@main def sum(n1: Int, n2: Int) =
+  val x1 = Number(n1)
+  val x2 = Number(n2)
+  val expr = Plus(x1, x2)
+  println(s"Expression: $expr")
+  val result = expr.eval()
+  println(s"Calculated: $result")
+
+////
+
+import scala.annotation.{ MainAnnotation, experimental }
+import scala.annotation.MainAnnotation.{ Info, Parameter }
+import scala.util.CommandLineParser.FromString
+
+@experimental class showAndEval extends MainAnnotation[FromString, Expression]:
+  def command(info: Info, args: Seq[String]): Option[Seq[String]] =
+    assert(info.parameters.forall(param => param.typeName == "Number"), "Only Number parameters allowed")
+    println(s"executing ${info.name} with inputs: ${args.mkString(" ")}")
+    Some(args)
+
+  def argGetter[T](param: Parameter, arg: String, defaultArgument: Option[() => T])(using parser: FromString[T]): () => T =
+    () => parser.fromString(arg)
+
+  def varargGetter[T](param: Parameter, args: Seq[String])(using parser: FromString[T]): () => Seq[T] =
+    () => args.map(arg => parser.fromString(arg))
+
+  def run(program: () => Expression): Unit =
+    val expr = program()
+    println(s"Expression: $expr")
+    val result = expr.eval()
+    println(s"Calculated: $result")
+end showAndEval
+
+given FromString[Number] = (x: String) => Number(x.toInt)
+
+////
+
+@showAndEval def sum2(x1: Number, x2: Number): Expression =
+  sumAll(x1, x2)
+
+@showAndEval def sumAll(xs: Number*): Expression =
+  if xs.isEmpty then Number(0)
+  else xs.tail.fold[Expression](xs.head)(Plus)
+
+////
+
+@main def Test: Unit =
+  def callMain(name: String, args: String*): Unit =
+    val clazz = Class.forName(name)
+    val method = clazz.getMethod("main", classOf[Array[String]])
+    method.invoke(null, args.toArray)
+  callMain("sum", "1", "2")
+  callMain("sum2", "2", "3")
+  callMain("sumAll", "1", "2", "3")
+end Test