SI-6771 Alias awareness for checkableType in match analysis.

Failure to dealias the type of the scrutinee led the pattern
matcher to incorrectly reason about the type test in:

   type Id[X] = X; (null: Id[Option[Int]]) match { case Some(_) => }

Before, `checkableType` returned `Id[?]`, now it returns `Some[?]`.

1 files changed, 9 insertions, 0 deletions

diff --git a/test/files/pos/t6771.scala b/test/files/pos/t6771.scala
new file mode 100644
index 0000000000..0f0bd4e4a0
--- /dev/null
+++ b/test/files/pos/t6771.scala
@@ -0,0 +1,9 @@
+object Test {
+  type Id[X] = X
+  val a: Id[Option[Int]] = None
+
+  a match {
+     case Some(x) => println(x)
+     case None    =>
+  }
+}