Fix #8031: Add testcase

tests/pos/i8031.scala

@@ -0,0 +1,38 @@
+object Example extends App {
+
+  trait Has[A]
+
+  trait ZIO[-R] {
+    def provideLayer[R0, R1 <: Has[_]](
+        layer: ZLayer[R0, R1]
+    )(implicit ev: R1 <:< R): ZIO[R0] =
+      ???
+  }
+
+  trait ZLayer[-RIn, +ROut <: Has[_]] {
+    def ++[RIn2, ROut1 >: ROut <: Has[_], ROut2 <: Has[_]](
+        that: ZLayer[RIn2, ROut2]
+    ): ZLayer[RIn with RIn2, ROut1 with ROut2] = ???
+  }
+
+  trait RandomService
+  trait SizedService
+
+  type Random = Has[RandomService]
+  type Sized = Has[SizedService]
+
+  def random: ZLayer[Random, Random] = ???
+  def sized: ZLayer[Any, Sized] = ???
+
+  lazy val zio: ZIO[Random with Sized] = ???
+
+  // Okay on Scala 2, does not compile on Dotty
+  lazy val eliminated: ZIO[Random] =
+    zio.provideLayer(random ++ sized)
+
+  // Compiles on Dotty with an explicit type annotation
+  lazy val eliminated2: ZIO[Random] =
+    zio.provideLayer[Random, Random with Sized](random ++ sized)
+
+  println("It compiles!")
+}