SI-2066 Plug a soundness hole higher order type params, overriding

PolyType-s parameterized by higher order type parameters (HOTPs)
should only be relatable with <:< or =:= if the variances of their
type parameters line up. This is only enforced for HOTPs defined in
method type arguments.

Invariant type parameters subsume variant ones. Concretely, as
described by @S11001001:

> A method taking [F[_]] can implement a method taking [F[+_]].
> Likewise, a method taking [F[_[+_]]] can implement a method
> taking [F[_[_]]], as with [F[_[_[_]]]] implementing [F[_[_[+_]]]],
> and so on, the variance subsumption flipping at each step.
>
> This is just the opposite of the variance for passing type
> parameters to either instantiate types or call methods; a F[+_]
> is a suitable F[_]-argument, a F[_[_]] a suitable F[_[+_]]-argument,
> and so on.
>
> All of the above rules can be duplicated for contravariant positions
> by substituting - for +. Also, something similar happens for
> weakening/strengthening bounds, I believe.

These cases are tested in the `// okay` lines in `neg/t2066.scala`.

1 files changed, 0 insertions, 16 deletions

diff --git a/test/pending/neg/t2066.scala b/test/pending/neg/t2066.scala
deleted file mode 100644
index 46177b19f7..0000000000
--- a/test/pending/neg/t2066.scala
+++ /dev/null
@@ -1,16 +0,0 @@
-object Test extends App {
-	trait A {
-	 def f[T[_]](x : T[Int]) : T[Any]
-	}
-
-	class B extends A {
-	 def f[T[+_]](x : T[Int]) : T[Any] = x
-	}
-
-	class P[Y](var y : Y)
-
-	val p = new P(1)
-	val palias = (new B():A).f[P](p)
-	palias.y = "hello"	
-	val z: Int = p.y
-}
\ No newline at end of file