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, 7 insertions, 0 deletions

diff --git a/test/files/neg/t7872b.check b/test/files/neg/t7872b.check
new file mode 100644
index 0000000000..0dc4e76301
--- /dev/null
+++ b/test/files/neg/t7872b.check
@@ -0,0 +1,7 @@
+t7872b.scala:8: error: contravariant type a occurs in covariant position in type [-a]List[a] of type l
+  def oops1 = down[({type l[-a] = List[a]})#l](List('whatever: Object)).head + "oops"
+                          ^
+t7872b.scala:19: error: covariant type a occurs in contravariant position in type [+a]coinv.Stringer[a] of type l
+  def oops2 = up[({type l[+a] = Stringer[a]})#l]("printed: " + _)
+                        ^
+two errors found