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  % cat test/pending/pos/t4717.scala
  trait Bounds[@specialized A] {
    // okay without `>: A`
    def x[B >: A]: Unit = new Bounds[B] {
      lazy val it = ???  // def or val okay
      it
    }
  }

  % scalac -d /tmp test/pending/pos/t4717.scala
  error:
       while compiling: test/pending/pos/t4717.scala
          during phase: specialize
       library version: version 2.10.0-20120510-134429-ce1d68ed19
      compiler version: version 2.10.0-20120510-152646-ba4dfd1e63
    reconstructed args: -d /tmp

    last tree to typer: Select(This(trait Bounds$mcZ$sp), x$mcZ$sp)
                symbol: method x$mcZ$sp in trait Bounds$mcZ$sp (flags: override <method> <specialized>)
     symbol definition: override def x$mcZ$sp[B >: Boolean](): Unit
                   tpe: [B >: Boolean]()Unit
         symbol owners: method x$mcZ$sp -> trait Bounds$mcZ$sp -> package <empty>
        context owners: value it -> anonymous class $anon -> method x$mcZ$sp -> trait Bounds$mcZ$sp -> package <empty>

  == Enclosing template or block ==

  Block(
    Assign(
      $anon.this."it "
      Apply( // def ???(): Nothing in object Predef, tree.tpe=Nothing
        scala.this."Predef"."$qmark$qmark$qmark" // def ???(): Nothing in object Predef, tree.tpe=()Nothing
        Nil
      )
    )
    $anon.this."it " // lazy private[this] var it: Nothing, tree.tpe=Nothing
  )

  == Expanded type of tree ==

  PolyType(
    typeParams = List(TypeParam(B >: Boolean))
    resultType = NullaryMethodType(
      resultType = TypeRef(TypeSymbol(final class Unit extends AnyVal))
    )
  )

  // And then the usual stack trace

1 files changed, 7 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 343636ff1e..34e1aaedfd 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -31,7 +31,6 @@ trait Typers extends Modes with Adaptations with Taggings {
 
   import global._
   import definitions._
-
   import patmat.DefaultOverrideMatchAttachment
 
   final def forArgMode(fun: Tree, mode: Int) =
@@ -85,6 +84,12 @@ trait Typers extends Modes with Adaptations with Taggings {
 
   private def isPastTyper = phase.id > currentRun.typerPhase.id
 
+  // To enable decent error messages when the typer crashes.
+  // TODO - this only catches trees which go through def typed,
+  // but there are all kinds of back ways - typedClassDef, etc. etc.
+  // Funnel everything through one doorway.
+  var lastTreeToTyper: Tree = EmptyTree
+
   // when true:
   //  - we may virtualize matches (if -Xexperimental and there's a suitable __match in scope)
   //  - we synthesize PartialFunction implementations for `x => x match {...}` and `match {...}` when the expected type is PartialFunction
@@ -4972,6 +4977,7 @@ trait Typers extends Modes with Adaptations with Taggings {
      *  @return     ...
      */
     def typed(tree: Tree, mode: Int, pt: Type): Tree = {
+      lastTreeToTyper = tree
       indentTyping()
 
       var alreadyTyped = false