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

2 files changed, 6 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/interpreter/IMain.scala b/src/compiler/scala/tools/nsc/interpreter/IMain.scala
index a55f0af116..bed8570bd0 100644
--- a/src/compiler/scala/tools/nsc/interpreter/IMain.scala
+++ b/src/compiler/scala/tools/nsc/interpreter/IMain.scala
@@ -262,7 +262,9 @@ class IMain(initialSettings: Settings, protected val out: JPrintWriter) extends
   protected def newCompiler(settings: Settings, reporter: Reporter): ReplGlobal = {
     settings.outputDirs setSingleOutput virtualDirectory
     settings.exposeEmptyPackage.value = true
-    new Global(settings, reporter) with ReplGlobal
+    new Global(settings, reporter) with ReplGlobal {
+      override def toString: String = "<global>"
+    }
   }
 
   /** Parent classloader.  Overridable. */
diff --git a/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala b/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
index 452957745a..4a765ea22e 100644
--- a/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/PatternMatching.scala
@@ -2818,7 +2818,9 @@ trait PatternMatching extends Transform with TypingTransformers with ast.TreeDSL
               // compare to the fully known type `tp` (modulo abstract types),
               // so that we can rule out stuff like: sealed trait X[T]; class XInt extends X[Int] --> XInt not valid when enumerating X[String]
               // however, must approximate abstract types in
-              val subTp       = appliedType(pre.memberType(sym), sym.typeParams.map(_ => WildcardType))
+
+              val memberType  = nestedMemberType(sym, pre, tpApprox.typeSymbol.owner)
+              val subTp       = appliedType(memberType, sym.typeParams.map(_ => WildcardType))
               val subTpApprox = typer.infer.approximateAbstracts(subTp) // TODO: needed?
               // patmatDebug("subtp"+(subTpApprox <:< tpApprox, subTpApprox, tpApprox))
               if (subTpApprox <:< tpApprox) Some(checkableType(subTp))