Disallow higher-kinded types that simulate general existential types

We cannot handle such types in general. So we now check that a hk application

   C[args]

where some of the arguments are wildcards does not have as a supertype
a hk application

   ([X] -> B)[args]

3 files changed, 62 insertions, 1 deletions

diff --git a/tests/neg/existentials.scala b/tests/neg/existentials.scala
new file mode 100644
index 000000000..4798504d9
--- /dev/null
+++ b/tests/neg/existentials.scala
@@ -0,0 +1,61 @@
+object TestList {
+
+  var x: ([X] -> List[List[X]])[_] = List(List(1)) // error: unreducible
+  var y: ([X] -> List[Seq[X]])[_] = List(List(1)) // error: unreducible
+
+  x = x
+  y = y
+  y = x
+
+  val h = x.head
+  val x1: List[_] = h
+
+  var z: List[_] = x
+
+}
+object TestSet {
+
+  var x: ([Y] -> Set[Set[Y]])[_] = Set(Set("a")) // error: unreducible
+  var y: ([Y] -> Set[Iterable[Y]])[_] = Set(Set("a")) // error: unreducible
+
+  x = x
+  y = y
+
+  val h = x.head
+  val h1: Set[_] = h
+
+  // val p = x.+ // infinite loop in implicit search
+
+  var z: Set[_] = x
+
+}
+class TestX {
+
+  class C[T](x: T) {
+    def get: T = x
+    def cmp: T => Boolean = (x == _)
+  }
+
+  val x: ([Y] -> C[C[Y]])[_] = new C(new C("a")) // error: unreducible
+
+  type CC[X] = C[C[X]]
+  val y: CC[_] = ??? // error: unreducible
+
+  type D[X] <: C[X]
+
+  type DD = [X] -> D[D[X]]
+  val z: DD[_] = ??? // error: unreducible
+
+  val g = x.get
+
+  val c = x.cmp
+}
+
+object Test6014 {
+  case class CC[T](key: T)
+  type Alias[T] = Seq[CC[T]]
+
+  def f(xs: Seq[CC[_]]) = xs map { case CC(x) => CC(x) }    // ok
+  def g(xs: Alias[_])   = xs map { case CC(x) => CC(x) }    // error: unreducible application
+}
+
diff --git a/tests/pos/GenTraversableFactory.scala b/tests/pos-scala2/GenTraversableFactory.scala
index 2f93ab27b..2f93ab27b 100644
--- a/tests/pos/GenTraversableFactory.scala
+++ b/tests/pos-scala2/GenTraversableFactory.scala
diff --git a/tests/pos/t6014.scala b/tests/pos-scala2/t6014.scala
index 26e258a27..02535f377 100644
--- a/tests/pos/t6014.scala
+++ b/tests/pos-scala2/t6014.scala
@@ -3,7 +3,7 @@ object Test {
   type Alias[T] = Seq[CC[T]]
 
   def f(xs: Seq[CC[_]]) = xs map { case CC(x) => CC(x) }    // ok
-  def g(xs: Alias[_])   = xs map { case CC(x) => CC(x) }    // fails
+  def g(xs: Alias[_])   = xs map { case CC(x) => CC(x) }    // migration warning: unreducible application
   // ./a.scala:11: error: missing parameter type for expanded function
   // The argument types of an anonymous function must be fully known. (SLS 8.5)
   // Expected type was: ?