Fix #3187: Don't crash in requiredClass if class is missing

Also, issue stacktraces for missing refs only if Y-debug-missing-refs is set.

The test case issues error messages without positions, so it is left in `pos`.

Author: odersky

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -77,6 +77,7 @@ class ScalaSettings extends Settings.SettingGroup {
   val Ydebug = BooleanSetting("-Ydebug", "Increase the quantity of debugging output.")
   val YdebugTrace = BooleanSetting("-Ydebug-trace", "Trace core operations")
   val YdebugFlags = BooleanSetting("-Ydebug-flags", "Print all flags of definitions")
+  val YdebugMissingRefs = BooleanSetting("-Ydebug-missing-refs", "Print a stacktrace when a required symbol is missing")
   val YdebugNames = BooleanSetting("-Ydebug-names", "Show internal representation of names")
   val YdebugOwners = BooleanSetting("-Ydebug-owners", "Print all owners of definitions (requires -Yprint-syms)")
   val YtermConflict = ChoiceSetting("-Yresolve-term-conflict", "strategy", "Resolve term conflicts", List("package", "object", "error"), "error")

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

@@ -277,7 +277,7 @@ object Denotations {
       disambiguate(p) match {
         case m @ MissingRef(ownerd, name) =>
           if (generateStubs) {
-            m.ex.printStackTrace()
+            if (ctx.settings.YdebugMissingRefs.value) m.ex.printStackTrace()
             ctx.newStubSymbol(ownerd.symbol, name, source)
           }
           else NoSymbol
@@ -1115,7 +1115,7 @@ object Denotations {
    *  Produced by staticRef, consumed by requiredSymbol.
    */
   case class MissingRef(val owner: SingleDenotation, name: Name)(implicit ctx: Context) extends ErrorDenotation {
-    val ex: Exception = new Exception
+    val ex: Exception = new Exception // DEBUG
   }
 
   /** An error denotation that provides more info about alternatives

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

@@ -369,7 +369,10 @@ trait Symbols { this: Context =>
   def requiredPackageRef(path: PreName): TermRef = requiredPackage(path).termRef
 
   def requiredClass(path: PreName): ClassSymbol =
-    base.staticRef(path.toTypeName).requiredSymbol(_.isClass).asClass
+    base.staticRef(path.toTypeName).requiredSymbol(_.isClass) match {
+      case cls: ClassSymbol => cls
+      case sym => defn.AnyClass
+    }
 
   def requiredClassRef(path: PreName): TypeRef = requiredClass(path).typeRef
 

tests/pos/i3187.scala

@@ -0,0 +1,3 @@
+package scala
+
+object collection