Assert we are loading TASTy with -Yscala2-stdlib

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -153,6 +153,7 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
   assert(moduleRoot.isTerm)
 
   checkVersion(using ictx)
+  checkScala2Stdlib(using ictx)
 
   private val loadingMirror = defn(using ictx) // was: mirrorThatLoaded(classRoot)
 
@@ -239,6 +240,9 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
         " in " + source)
   }
 
+  private def checkScala2Stdlib(using Context): Unit =
+    assert(!ctx.settings.Yscala2Stdlib.value, "No Scala 2 libraries should be unpickled under -Yscala2-stdlib")
+
   /** The `decls` scope associated with given symbol */
   protected def symScope(sym: Symbol): Scope = symScopes.getOrElseUpdate(sym, newScope(0))