Wrong type inferred for Array ++

[nicolasstucki]

The following code compiles under scalac but not in dotty.

class Test {
  class Foo[+T](val x: T)

  def unfoo[T](foo: Foo[T]): T = foo.x

  def fooArray: Foo[Array[Object]] = new Foo(Array.empty[Object])

  unfoo(fooArray) ++ unfoo(fooArray)
}

It infers an imprecise type for one of the type argument of the second unfoo

-- [E007] Type Mismatch Error: foo.scala:9:27 --------
9 |  unfoo(fooArray) ++ unfoo(fooArray)
  |                           ^^^^^^^^
  |         Found:    Test.this.Foo[Array[Object]]
  |         Required: Test.this.Foo[scala.collection.GenTraversableOnce[Any]]

result of foo.scala after frontend:
package <empty> {
  class Test() extends Object() { 
    class Foo[T >: Nothing <: Any](x: T) extends Object() { 
       +T
      val x: T
    }
    def unfoo[T >: Nothing <: Any](foo: Test.this.Foo[T]): T = foo.x
    def fooArray: Test.this.Foo[Array[Object]] = 
      new Test.this.Foo[Array[Object]](
        Array.empty[Object](
          scala.reflect.ClassTag.apply[Object](classOf[Object])
        )
      )
    refArrayOps[Object](this.unfoo[Array[Object]](this.fooArray)).++[Object, 
      Array[Object]
    ](this.unfoo[scala.collection.GenTraversableOnce[Object]](this.fooArray))(
      Array.canBuildFrom[Object](
        scala.reflect.ClassTag.apply[Object](classOf[Object])
      )
    )
  }
}