Temporary hack to circumvent missing sourcefile info

Will be no longer necessary once bootstrap library uses new Tasty format.

Author: odersky

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

@@ -112,9 +112,16 @@ class TreeUnpickler(reader: TastyReader,
     val owner = ctx.owner
     val source = ctx.source
     def complete(denot: SymDenotation)(implicit ctx: Context): Unit = {
+      val sourceToUse = if (source.exists) source else ctx.source
+        // TODO: remove. This is a hack to get pickling tests (notable tuple-cons-2.scala
+        // to pass. The problem is that without a bootstrapped library some unpicked
+        // files are lacking a source, and if these are completed from Definitions
+        // there's also no external source given in `context`. In that case we switch
+        // late and pick the calling context's source. We can drop this hack once
+        // all Tasty trees have a SOURCE entry, which will determine the context source
       treeAtAddr(currentAddr) =
         new TreeReader(reader).readIndexedDef()(
-          ctx.withPhaseNoLater(ctx.picklerPhase).withOwner(owner).withSource(source))
+          ctx.withPhaseNoLater(ctx.picklerPhase).withOwner(owner).withSource(sourceToUse))
     }
   }