stripTypeVars after fullyDefinedType

compiler/src/dotty/tools/dotc/typer/Synthesizer.scala

@@ -541,7 +541,9 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
     if checkFormal(formal) then
       formal.member(tpnme.MirroredType).info match
         case TypeBounds(mirroredType, _) =>
-          synth(fullyDefinedType(mirroredType, "Mirror.*Of argument", ctx.source.atSpan(span)), formal, span)
+          val defined = fullyDefinedType(mirroredType, "Mirror.*Of argument", ctx.source.atSpan(span))
+          val stripped = TypeOps.stripTypeVars(defined)
+          synth(stripped, formal, span)
         case other => EmptyTreeNoError
     else EmptyTreeNoError
 

tests/run/i13146poly.scala

@@ -0,0 +1,19 @@
+import scala.deriving.*
+
+trait Functor[F[_]]
+
+object Functor:
+  given [C]: Functor[[T] =>> C]()
+  given Functor[[T] =>> Tuple1[T]]()
+  given t2 [T]: Functor[[U] =>> (T, U)]()
+  given t3 [T, U]: Functor[[V] =>> (T, U, V)]()
+
+  def derived[F[_]](using m: Mirror { type MirroredType[X] = F[X] ; type MirroredElemTypes[_] }, r: Functor[m.MirroredElemTypes]): Functor[F] = new Functor[F] {}
+
+case class Mono(i: Int) derives Functor
+case class Poly[A](a: A) derives Functor
+//case class Poly11[F[_]](fi: F[Int]) derives Functor
+case class Poly2[A, B](a: A, b: B) derives Functor
+case class Poly3[A, B, C](a: A, b: B, c: C) derives Functor
+
+@main def Test = ()