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.

Author: odersky

src/dotty/tools/dotc/typer/Typer.scala

@@ -844,8 +844,9 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
   }
 
   def typedBind(tree: untpd.Bind, pt: Type)(implicit ctx: Context): Bind = track("typedBind") {
-    val body1 = typed(tree.body, pt)
-    typr.println(i"typed bind $tree pt = $pt bodytpe = ${body1.tpe}")
+    val pt1 = fullyDefinedType(pt, "pattern variable", tree.pos)
+    val body1 = typed(tree.body, pt1)
+    typr.println(i"typed bind $tree pt = $pt1 bodytpe = ${body1.tpe}")
     val flags = if (tree.isType) BindDefinedType else EmptyFlags
     val sym = ctx.newSymbol(ctx.owner, tree.name, flags, body1.tpe, coord = tree.pos)
     assignType(cpy.Bind(tree)(tree.name, body1), sym)

tests/pos/Patterns.scala

@@ -94,3 +94,8 @@ object Patterns {
       t
   }
 }
+
+object NestedPattern {
+  val xss: List[List[String]] = ???
+  val List(List(x)) = xss
+}