Fix the source of inlined trees after Erasure

To ensure that the sources change, the trees need to be copied with the
new source rather than preserving it.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/Driver.scala

@@ -6,7 +6,7 @@ import dotty.tools.FatalError
 import config.CompilerCommand
 import core.Comments.{ContextDoc, ContextDocstrings}
 import core.Contexts.{Context, ContextBase}
-import core.Mode
+import core.{Mode, TypeError}
 import reporting._
 
 import scala.util.control.NonFatal
@@ -37,6 +37,9 @@ class Driver {
         case ex: FatalError  =>
           ctx.error(ex.getMessage) // signals that we should fail compilation.
           ctx.reporter
+        case ex: TypeError =>
+          println(s"${ex.toMessage} while compiling ${fileNames.mkString(", ")}")
+          throw ex
         case ex: Throwable =>
           println(s"$ex while compiling ${fileNames.mkString(", ")}")
           throw ex

compiler/src/dotty/tools/dotc/ast/Positioned.scala

@@ -113,6 +113,9 @@ abstract class Positioned(implicit @constructorOnly src: SourceFile) extends Pro
 
   /** Clone this node but assign it a fresh id which marks it as a node in `file`. */
   def cloneIn(src: SourceFile): this.type = {
+    assert(this ne tpd.EmptyTree)
+    assert(this ne tpd.EmptyValDef)
+    assert(this ne tpd.ContextualEmptyTree)
     val newpd: this.type = clone.asInstanceOf[this.type]
     newpd.uniqueId = src.nextId
     newpd