Make sure types of pattern bound variables are fully-defined.

Like all other variables, pattern-bound vars need a fully defined type. I was
thinking originally that demanding a fully defined selector type is sufficient
to ensure this, but that's not true: An outer pattern might call a polymorphic
unapply and its type variables need not be fully instantiated.

With the fix, the minimized test case from ExpandSAMs works.

1 files changed, 5 insertions, 0 deletions

diff --git a/tests/pos/Patterns.scala b/tests/pos/Patterns.scala
index e443c2ab5..e9bce87a9 100644
--- a/tests/pos/Patterns.scala
+++ b/tests/pos/Patterns.scala
@@ -94,3 +94,8 @@ object Patterns {
       t
   }
 }
+
+object NestedPattern {
+  val xss: List[List[String]] = ???
+  val List(List(x)) = xss
+}