Fix scala#1905: Duplicate bridge

The problem with scala#1905 was that we checked "new" overriding relationships
are phase erasure + 1. This is wrong when we have ErasedValueTypes because
these do not compare correctly ith their underlying type. So we get spurious
mismatches which force in turn spurious bridge generation. The fix is to
compute newly overridden symbols at phase elimErasedValueTypes + 1, i.e. one
phase later.

Author: odersky

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -600,6 +600,7 @@ object Erasure extends TypeTestsCasts{
     // this implementation doesn't check for bridge clashes with value types!
     def addBridges(oldStats: List[untpd.Tree], newStats: List[tpd.Tree])(implicit ctx: Context): List[tpd.Tree] = {
       val beforeCtx = ctx.withPhase(ctx.erasurePhase)
+      val afterCtx = ctx.withPhase(ctx.elimErasedValueTypePhase.next)
       def traverse(after: List[Tree], before: List[untpd.Tree],
                    emittedBridges: ListBuffer[tpd.DefDef] = ListBuffer[tpd.DefDef]()): List[tpd.DefDef] = {
         after match {
@@ -613,7 +614,7 @@ object Erasure extends TypeTestsCasts{
                   val newSymbol = member.symbol(ctx)
                   assert(oldSymbol.name(beforeCtx) == newSymbol.name,
                     s"${oldSymbol.name(beforeCtx)} bridging with ${newSymbol.name}")
-                  val newOverridden = oldSymbol.denot.allOverriddenSymbols.toSet // TODO: clarify new <-> old in a comment; symbols are swapped here
+                  val newOverridden = oldSymbol.denot.allOverriddenSymbols(afterCtx).toSet // TODO: clarify new <-> old in a comment; symbols are swapped here
                   val oldOverridden = newSymbol.allOverriddenSymbols(beforeCtx).toSet // TODO: can we find a more efficient impl? newOverridden does not have to be a set!
                   def stillInBaseClass(sym: Symbol) = ctx.owner derivesFrom sym.owner
                   val neededBridges = (oldOverridden -- newOverridden).filter(stillInBaseClass)