Drop restriction that SuperAccessors should not touch patterns

Not sure why we need to do this, and in any case it's not sure
what constitutes a pattern. There are certainly some parts of
patterns (e.g. prefixes of unapplies, or their implicit arguments)
that should be transformed under SuperAccessors, so the previous
condition was too coarse.

We include the test case that motivated the restriction. It looks like
it works now.

Author: odersky

src/dotty/tools/dotc/transform/SuperAccessors.scala

@@ -155,11 +155,6 @@ class SuperAccessors extends MacroTransform
         else tree
 
       try tree match {
-        // Don't transform patterns or strange trees will reach the matcher (ticket #4062)
-        // TODO Query `ctx.mode is Pattern` instead.
-        case CaseDef(pat, guard, body) =>
-          cpy.CaseDef(tree)(pat, transform(guard), transform(body))
-
         case impl: Template =>
 
           def transformTemplate = {

tests/pos/t4062.scala

@@ -0,0 +1,7 @@
+class A(val f : String)
+
+class B(a : Option[String], f : String) extends A(f) {
+  a match {
+    case Some(`f`) => print(f)
+  }
+}