Add regression test

Closes #16420

tests/pos-macros/i16420/Macro.scala

@@ -0,0 +1,21 @@
+import scala.quoted.{Expr, Quotes, Type}
+
+object Converter {
+  private def handleUnit[R](f: Expr[Int ?=> R])(using q: Quotes, rt: Type[R]): Expr[Unit] = '{}
+
+  class UnitConverter[R] extends Converter[EmptyTuple, R, Int ?=> R] {
+    inline def convert(inline f: Int ?=> R): Unit = ${ handleUnit[R]('f) }
+  }
+
+  inline given unitHandler[R]: UnitConverter[R] = new UnitConverter[R]
+}
+
+
+trait Converter[T <: Tuple, R, F] {
+  inline def convert(inline fn: F): Unit
+}
+
+abstract class Directive[R <: Tuple] {
+  inline def apply[O, F](using inline c: Converter[R, O, F])(inline fn: F): Unit =
+    c.convert(fn)
+}

tests/pos-macros/i16420/Test.scala

@@ -0,0 +1,8 @@
+object Meow extends App {
+  case class Meow(s: String, i: Int)
+
+  val dir: Directive[EmptyTuple] = ???
+  dir {
+    Meow("asd", 123)
+  }
+}