Fix: Preserve mode when lazy unpickling

In readLater the context is not preserved, but reconstituted using the
owner of the context building the closure. We need to pass the mode as
well, or else ReadPositions might be reset. I noticed this when I saw
that positions were not read correctly when inlining.

Author: odersky

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -1135,7 +1135,7 @@ class TreeUnpickler(reader: TastyReader,
     def readLater[T <: AnyRef](end: Addr, op: TreeReader => Context => T)(implicit ctx: Context): Trees.Lazy[T] = {
       val localReader = fork
       goto(end)
-      new LazyReader(localReader, ctx.owner, op)
+      new LazyReader(localReader, ctx.owner, ctx.mode, op)
     }
 
     def readHole(end: Addr, isType: Boolean)(implicit ctx: Context): Tree = {
@@ -1182,10 +1182,10 @@ class TreeUnpickler(reader: TastyReader,
     }
   }
 
-  class LazyReader[T <: AnyRef](reader: TreeReader, owner: Symbol, op: TreeReader => Context => T) extends Trees.Lazy[T] {
+  class LazyReader[T <: AnyRef](reader: TreeReader, owner: Symbol, mode: Mode, op: TreeReader => Context => T) extends Trees.Lazy[T] {
     def complete(implicit ctx: Context): T = {
       pickling.println(i"starting to read at ${reader.reader.currentAddr} with owner $owner")
-      op(reader)(ctx.withPhaseNoLater(ctx.picklerPhase).withOwner(owner))
+      op(reader)(ctx.withPhaseNoLater(ctx.picklerPhase).withOwner(owner).withModeBits(mode))
     }
   }