Test for #13001

Author: odersky

tests/pos/i13001/Main_1.scala

@@ -0,0 +1,31 @@
+case class Foo(a: String)
+
+trait Arbitrary[T]
+trait Gen[+T]
+
+object ArbitraryDerivation:
+  given deriveArb[A](using gen: DerivedGen[A]): Arbitrary[A] = ???
+
+opaque type DerivedGen[A] = Gen[A]
+object DerivedGen extends DerivedGenInstances
+
+sealed abstract class DerivedGenInstances:
+  inline given derived[A](using gen: K0.Generic[A]): DerivedGen[A] =
+    val dummy: DerivedGen[A] = ???
+    gen.derive(dummy, dummy)
+
+// from shapeless3-deriving
+import scala.deriving.*
+object K0 {
+  type Kind[C, O] = C { type Kind = K0.type ; type MirroredType = O ; type MirroredElemTypes <: Tuple }
+  type Generic[O] = Kind[Mirror, O]
+  type ProductGeneric[O] = Kind[Mirror.Product, O]
+  type CoproductGeneric[O] = Kind[Mirror.Sum, O]
+
+  extension [F[_], T](gen: Generic[T])
+    inline def derive(f: => (ProductGeneric[T] & gen.type) ?=> F[T], g: => (CoproductGeneric[T] & gen.type) ?=> F[T]): F[T] =
+      inline gen match {
+        case p: ProductGeneric[T]   => f(using p.asInstanceOf)
+        case c: CoproductGeneric[T] => g(using c.asInstanceOf)
+      }
+}

tests/pos/i13001/Test_2.scala

@@ -0,0 +1,4 @@
+class Test:
+  import ArbitraryDerivation.given
+  private def test[A: Arbitrary]: Unit = {}
+  test[Foo]