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, 10 insertions, 0 deletions

diff --git a/test/files/neg/t7872.check b/test/files/neg/t7872.check
new file mode 100644
index 0000000000..57d9772abc
--- /dev/null
+++ b/test/files/neg/t7872.check
@@ -0,0 +1,10 @@
+t7872.scala:6: error: contravariant type a occurs in covariant position in type [-a]Cov[a] of type l
+  type x = {type l[-a] = Cov[a]}
+                 ^
+t7872.scala:8: error: covariant type a occurs in contravariant position in type [+a]Inv[a] of type l
+  foo[({type l[+a] = Inv[a]})#l]
+             ^
+t7872.scala:5: error: contravariant type a occurs in covariant position in type [-a]Cov[a] of type l
+  type l[-a] = Cov[a]
+       ^
+three errors found