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]

1 files changed, 13 insertions, 0 deletions

diff --git a/tests/pos-scala2/t6014.scala b/tests/pos-scala2/t6014.scala
new file mode 100644
index 000000000..02535f377
--- /dev/null
+++ b/tests/pos-scala2/t6014.scala
@@ -0,0 +1,13 @@
+object Test {
+  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) }    // 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: ?
+  //   def g(xs: Alias[_])   = xs map { case CC(x) => CC(x) }    // fails
+  //                                  ^
+  // one error found
+}