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, 11 insertions, 0 deletions

diff --git a/test/files/neg/t7872c.check b/test/files/neg/t7872c.check
new file mode 100644
index 0000000000..469449dbd5
--- /dev/null
+++ b/test/files/neg/t7872c.check
@@ -0,0 +1,11 @@
+t7872c.scala:7: error: inferred kinds of the type arguments (List) do not conform to the expected kinds of the type parameters (type F).
+List's type parameters do not match type F's expected parameters:
+type A is covariant, but type _ is declared contravariant
+  down(List('whatever: Object))
+  ^
+t7872c.scala:7: error: type mismatch;
+ found   : List[Object]
+ required: F[Object]
+  down(List('whatever: Object))
+           ^
+two errors found