SI-6146 More accurate prefixes for sealed subtypes.

When analysing exhaustivity/reachability of type tests
and equality tests, the pattern matcher must construct
a set of sealed subtypes based on the prefix of the
static type of and the set of sealed descendent symbols
of that type.

Previously, it was using `memberType` for this purpose.
In simple cases, this is sufficient:

    scala> class C { class I1; object O { class I2 } }; object D extends C
    defined class C
    defined module D

    scala> typeOf[D.type] memberType typeOf[C#I1].typeSymbol
    res0: u.Type = D.I1

But, as reported in this bug, it fails when there is an
additional level of nesting:

    scala> typeOf[D.type] memberType typeOf[c.O.I2 forSome { val c: C }].typeSymbol
    res5: u.Type = C.O.I2

This commit introduces `nestedMemberType`, which uses `memberType`
recursively up the prefix chain prefix chain.

    scala> nestedMemberType(typeOf[c.O.I2 forSome { val c: C }].typeSymbol, typeOf[D.type], typeOf[C].typeSymbol)
    res6: u.Type = D.O.Id

1 files changed, 60 insertions, 0 deletions

diff --git a/test/files/pos/t6146.scala b/test/files/pos/t6146.scala
new file mode 100644
index 0000000000..b5bde826b1
--- /dev/null
+++ b/test/files/pos/t6146.scala
@@ -0,0 +1,60 @@
+// No unreachable or exhaustiveness warnings, please.
+
+//
+// The reported bug
+//
+
+trait AxisCompanion {
+   sealed trait Format
+   object Format {
+      case object Decimal extends Format
+      case object Integer extends Format
+      // Gives an unrelated warning:  The outer reference in this type test cannot be checked at run time.
+      //final case class Time( hours: Boolean = false, millis: Boolean = true ) extends Format
+   }
+}
+object Axis extends AxisCompanion
+class Axis {
+   import Axis._
+   def test( f: Format ) = f match {
+      case Format.Integer => "Int"
+      // case Format.Time( hours, millis ) => "Time"
+      case Format.Decimal => "Dec"
+   }
+}
+
+
+//
+// Some tricksier variations
+//
+
+trait T1[X] {
+  trait T2[Y] {
+    sealed trait Format
+    object Format {
+      case object Decimal extends Format
+      case object Integer extends Format
+    }
+  }
+}
+
+object O1 extends T1[Any] {
+  object O2 extends T2[Any] {
+
+  }
+}
+
+case object Shorty extends O1.O2.Format
+
+class Test1 {
+   import O1.O2._
+   val FI: Format.Integer.type = Format.Integer
+   def test( f: Format ) = {
+     val ff: f.type = f
+     ff match {
+        case FI => "Int"
+        case Format.Decimal => "Dec"
+        case Shorty => "Sho"
+     }
+   }
+}