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.

7 files changed, 72 insertions, 1 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
diff --git a/test/files/neg/t7872.scala b/test/files/neg/t7872.scala
new file mode 100644
index 0000000000..66d22a0715
--- /dev/null
+++ b/test/files/neg/t7872.scala
@@ -0,0 +1,9 @@
+trait Cov[+A]
+trait Inv[-A]
+
+object varianceExploit {  
+  type l[-a] = Cov[a]
+  type x = {type l[-a] = Cov[a]}
+  def foo[M[_]] = ()
+  foo[({type l[+a] = Inv[a]})#l]
+}
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
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)
+}
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
diff --git a/test/files/neg/t7872c.scala b/test/files/neg/t7872c.scala
new file mode 100644
index 0000000000..fa12a523b5
--- /dev/null
+++ b/test/files/neg/t7872c.scala
@@ -0,0 +1,8 @@
+object coinv {
+  def up[F[+_]](fa: F[String]): F[Object] = fa
+  def down[F[-_]](fa: F[Object]): F[String] = fa
+ 
+  up(List("hi"))
+  // [error] type A is covariant, but type _ is declared contravariant
+  down(List('whatever: Object))
+}
diff --git a/test/files/neg/variances.check b/test/files/neg/variances.check
index 7d965e94dc..cb1a60a632 100644
--- a/test/files/neg/variances.check
+++ b/test/files/neg/variances.check
@@ -1,6 +1,9 @@
 variances.scala:4: error: covariant type A occurs in contravariant position in type test.Vector[A] of value x
   def append(x: Vector[A]): Vector[A]
              ^
+variances.scala:75: error: covariant type A occurs in contravariant position in type => A => A of value m
+      val m: A => A
+          ^
 variances.scala:18: error: covariant type A occurs in contravariant position in type A of value a
     private def setA3(a : A) = this.a = a
                       ^
@@ -19,4 +22,4 @@ variances.scala:89: error: covariant type T occurs in invariant position in type
 variances.scala:90: error: covariant type T occurs in contravariant position in type => test.TestAlias.B[C.this.A] of method foo
     def foo: B[A]
         ^
-7 errors found
+8 errors found