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

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

@@ -235,7 +235,6 @@ trait Symbols { this: Context =>
       case name: TypeName =>
         newClassSymbol(normalizedOwner, name, EmptyFlags, stubCompleter, assocFile = file)
     }
-    stubs = stub :: stubs
     stub
   }
 
@@ -369,7 +368,10 @@ 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
 
@@ -692,8 +694,6 @@ object Symbols {
   /** The current class */
   def currentClass(implicit ctx: Context): ClassSymbol = ctx.owner.enclosingClass.asClass
 
-  @sharable var stubs: List[Symbol] = Nil // diagnostic only
-
   /* Mutable map from symbols any T */
   class MutableSymbolMap[T](private[Symbols] val value: java.util.IdentityHashMap[Symbol, T]) extends AnyVal {
 

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

@@ -179,7 +179,7 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
     val ex = new BadSignature(
       i"""error reading Scala signature of $classRoot from $source:
          |error occurred at position $readIndex: $msg""")
-    if (ctx.debug || true) original.getOrElse(ex).printStackTrace() // temporarily enable printing of original failure signature to debug failing builds
+    if (ctx.settings.YdebugMissingRefs.value) original.getOrElse(ex).printStackTrace()
     throw ex
   }
 
@@ -415,7 +415,7 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
               owner.info.decls.checkConsistent()
               if (slowSearch(name).exists)
                 System.err.println(i"**** slow search found: ${slowSearch(name)}")
-              if (ctx.debug) Thread.dumpStack()
+              if (ctx.settings.YdebugMissingRefs.value) Thread.dumpStack()
               ctx.newStubSymbol(owner, name, source)
             }
           }

tests/neg/i3187.scala

@@ -0,0 +1,23 @@
+package scala
+
+// nopos-error
+// nopos-error
+// nopos-error
+// nopos-error
+// nopos-error
+// nopos-error
+// nopos-error
+// nopos-error
+// nopos-error
+// nopos-error
+
+// nopos-error
+// nopos-error
+// nopos-error
+// nopos-error
+// nopos-error
+// nopos-error
+// nopos-error
+// nopos-error
+
+object collection