Fix #2395: Check if the scala or dotty libraries are missing

Author: nicolasstucki

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

@@ -0,0 +1,9 @@
+package dotty.tools.dotc
+
+import dotty.tools.FatalError
+
+class MissingCoreLibraryException(rootPackage: String) extends FatalError(
+  s"""Could not find package $rootPackage from compiler core libraries.
+     |Make sure the compiler core libraries are on the classpath.
+   """.stripMargin
+)

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -212,7 +212,7 @@ class Definitions {
    *  in `scalaShadowing` so they don't clash with the same-named `scala`
    *  members at runtime.
    */
-  lazy val ScalaShadowingPackageVal = ctx.requiredPackage("scalaShadowing")
+  lazy val ScalaShadowingPackageVal = ctx.requiredPackage(nme.scalaShadowing)
   lazy val ScalaShadowingPackageClass = ScalaShadowingPackageVal.moduleClass.asClass
 
   /** Note: We cannot have same named methods defined in Object and Any (and AnyVal, for that matter)

compiler/src/dotty/tools/dotc/core/Denotations.scala

@@ -1199,7 +1199,10 @@ object Denotations {
             val alt =
               if (generateStubs) missingHook(owner.symbol.moduleClass, selector)
               else NoSymbol
-            if (alt.exists) alt.denot else MissingRef(owner, selector)
+            if (alt.exists) alt.denot
+            else if (owner.symbol == defn.RootClass && (selector == nme.scala_ || selector == nme.scalaShadowing))
+              throw new MissingCoreLibraryException(selector.toString)
+            else MissingRef(owner, selector)
           }
         }
         else owner

compiler/src/dotty/tools/dotc/core/StdNames.scala

@@ -491,6 +491,7 @@ object StdNames {
     val runtimeMirror: N        = "runtimeMirror"
     val sameElements: N         = "sameElements"
     val scala_ : N              = "scala"
+    val scalaShadowing : N      = "scalaShadowing"
     val selectDynamic: N        = "selectDynamic"
     val selectDynamicMethod: N  = "selectDynamicMethod"
     val selectOverloadedMethod: N = "selectOverloadedMethod"