SI-7872 Plug a variance exploit in refinement types

Refinement types are collapsed to a TypeTree with an original
during type checking; this was enough to evade variance validation
in refchecks.

This commit:

  - validates the original of `TypeTree`s in refchecks
  - changes VarianceValidator to recurse into:
    - the originals of `TypeTree`s
    - `TypTree` (to cover, e.g. `CompoundTypeTree` / `SelectFromTypeTree`)

It also finds an unreported variance violation in an existing
test case, variances.scala. This looks to be legitimate.

1 files changed, 23 insertions, 0 deletions

diff --git a/test/files/neg/t7872b.scala b/test/files/neg/t7872b.scala
new file mode 100644
index 0000000000..307a1470c5
--- /dev/null
+++ b/test/files/neg/t7872b.scala
@@ -0,0 +1,23 @@
+object coinv {
+  def up[F[+_]](fa: F[String]): F[Object] = fa
+  def down[F[-_]](fa: F[Object]): F[String] = fa
+ 
+  up(List("hi"))
+ 
+  // should not compile; `l' is unsound
+  def oops1 = down[({type l[-a] = List[a]})#l](List('whatever: Object)).head + "oops"
+  // scala> oops1
+  // java.lang.ClassCastException: scala.Symbol cannot be cast to java.lang.String
+  //         at com.nocandysw.coinv$.oops1(coinv.scala:12)
+ 
+  type Stringer[-A] = A => String
+  down[Stringer](_.toString)
+  // [error] type A is contravariant, but type _ is declared covariant
+  // up[Stringer]("printed: " + _)
+ 
+  // should not compile; `l' is unsound
+  def oops2 = up[({type l[+a] = Stringer[a]})#l]("printed: " + _)
+  // scala> oops2(Some(33))
+  // java.lang.ClassCastException: scala.Some cannot be cast to java.lang.String
+  //         at com.nocandysw.coinv$$anonfun$oops2$1.apply(coinv.scala:20)
+}