Merge pull request #4385 from dotty-staging/fix-#4368

Fix #4368: Avoid stackoverflows for some cyclic definitions

Author: Blaisorblade

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

@@ -184,11 +184,6 @@ object Config {
    */
   final val LogPendingFindMemberThreshold = 9
 
-  /** Maximal number of outstanding recursive calls to findMember before backing out
-   *  when findMemberLimit is set.
-   */
-  final val PendingFindMemberLimit = LogPendingFindMemberThreshold * 4
-
   /** When in IDE, turn StaleSymbol errors into warnings instead of crashing */
   final val ignoreStaleInIDE = true
 }

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

@@ -63,7 +63,7 @@ class ScalaSettings extends Settings.SettingGroup {
   val Xhelp = BooleanSetting("-X", "Print a synopsis of advanced options.")
   val XnoForwarders = BooleanSetting("-Xno-forwarders", "Do not generate static forwarders in mirror classes.")
   val XminImplicitSearchDepth = IntSetting("-Xmin-implicit-search-depth", "Set number of levels of implicit searches undertaken before checking for divergence.", 5)
-  val xmaxInlines = IntSetting("-Xmax-inlines", "Maximal number of successive inlines", 32)
+  val XmaxInlines = IntSetting("-Xmax-inlines", "Maximal number of successive inlines", 32)
   val XmaxClassfileName = IntSetting("-Xmax-classfile-name", "Maximum filename length for generated classes", 255, 72 to 255)
   val Xmigration = VersionSetting("-Xmigration", "Warn about constructs whose behavior may have changed since version.")
   val Xprint = PhasesSetting("-Xprint", "Print out program after")
@@ -146,6 +146,7 @@ class ScalaSettings extends Settings.SettingGroup {
   val Xlink = BooleanSetting("-Xlink", "Recompile library code with the application.")
   val YoptPhases = PhasesSetting("-Yopt-phases", "Restrict the optimisation phases to execute under -optimise.")
   val YoptFuel = IntSetting("-Yopt-fuel", "Maximum number of optimisations performed under -optimise.", -1)
+  val YnoDecodeStacktraces = BooleanSetting("-Yno-decode-stacktraces", "Show raw StackOverflow stacktraces, instead of decoding them into triggering operations.")
 
   /** Dottydoc specific settings */
   val siteRoot = StringSetting(

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

@@ -332,21 +332,8 @@ object Denotations {
     }
 
     /** Handle merge conflict by throwing a `MergeError` exception */
-    private def mergeConflict(tp1: Type, tp2: Type, that: Denotation)(implicit ctx: Context): Type = {
-      def showSymbol(sym: Symbol): String = if (sym.exists) sym.showLocated else "[unknown]"
-      def showType(tp: Type) = tp match {
-        case ClassInfo(_, cls, _, _, _) => cls.showLocated
-        case bounds: TypeBounds => i"type bounds $bounds"
-        case _ => tp.show
-      }
-      val msg =
-        s"""cannot merge
-           |  ${showSymbol(this.symbol)} of type ${showType(tp1)}  and
-           |  ${showSymbol(that.symbol)} of type ${showType(tp2)}
-           """
-      if (true) throw new MergeError(msg, tp1, tp2)
-      else throw new Error(msg) // flip condition for debugging
-    }
+    private def mergeConflict(tp1: Type, tp2: Type, that: Denotation)(implicit ctx: Context): Type =
+      throw new MergeError(this.symbol, that.symbol, tp1, tp2, NoPrefix)
 
     /** Merge parameter names of lambda types. If names in corresponding positions match, keep them,
      *  otherwise generate new synthetic names.
@@ -537,8 +524,7 @@ object Denotations {
                   else if (pre.widen.classSymbol.is(Scala2x) || ctx.scala2Mode)
                     info1 // follow Scala2 linearization -
                   // compare with way merge is performed in SymDenotation#computeMembersNamed
-                  else
-                    throw new MergeError(s"${ex.getMessage} as members of type ${pre.show}", ex.tp1, ex.tp2)
+                  else throw new MergeError(ex.sym1, ex.sym2, ex.tp1, ex.tp2, pre)
               }
             new JointRefDenotation(sym, jointInfo, denot1.validFor & denot2.validFor)
           }
@@ -1136,21 +1122,15 @@ object Denotations {
   def doubleDefError(denot1: Denotation, denot2: Denotation, pre: Type = NoPrefix)(implicit ctx: Context): Nothing = {
     val sym1 = denot1.symbol
     val sym2 = denot2.symbol
-    def fromWhere = if (pre == NoPrefix) "" else i"\nwhen seen as members of $pre"
-    val msg =
-      if (denot1.isTerm)
-        i"""cannot merge
-           |  $sym1: ${sym1.info}  and
-           |  $sym2: ${sym2.info};
-           |they are both defined in ${sym1.owner} but have matching signatures
-           |  ${denot1.info} and
-           |  ${denot2.info}$fromWhere"""
-      else
-        i"""cannot merge
-           |  $sym1 ${denot1.info}
-           |  $sym2 ${denot2.info}
-           |they are conflicting definitions$fromWhere"""
-    throw new MergeError(msg, denot2.info, denot2.info)
+    if (denot1.isTerm)
+      throw new MergeError(sym1, sym2, sym1.info, sym2.info, pre) {
+        override def addendum(implicit ctx: Context) =
+          i"""
+             |they are both defined in ${sym1.owner} but have matching signatures
+             |  ${denot1.info} and
+             |  ${denot2.info}${super.addendum}"""
+        }
+    else throw new MergeError(sym1, sym2, denot1.info, denot2.info, pre)
   }
 
   // --- Overloaded denotations and predenotations -------------------------------------------------

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

@@ -168,7 +168,7 @@ class TypeApplications(val self: Type) extends AnyVal {
    *  any type parameter that is-rebound by the refinement.
    */
   final def typeParams(implicit ctx: Context): List[TypeParamInfo] = /*>|>*/ track("typeParams") /*<|<*/ {
-    self match {
+    try self match {
       case self: TypeRef =>
         val tsym = self.symbol
         if (tsym.isClass) tsym.typeParams
@@ -193,6 +193,9 @@ class TypeApplications(val self: Type) extends AnyVal {
       case _ =>
         Nil
     }
+    catch {
+      case ex: Throwable => handleRecursive("type parameters of", self.show, ex)
+    }
   }
 
   /** If `self` is a higher-kinded type, its type parameters, otherwise Nil */

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

@@ -121,7 +121,11 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
   protected def isSubType(tp1: Type, tp2: Type, a: ApproxState): Boolean = {
     val saved = approx
     this.approx = a
-    try recur(tp1, tp2) finally this.approx = saved
+    try recur(tp1, tp2)
+    catch {
+      case ex: Throwable => handleRecursive("subtype", i"$tp1 <:< $tp2", ex, weight = 2)
+    }
+    finally this.approx = saved
   }
 
   protected def isSubType(tp1: Type, tp2: Type): Boolean = isSubType(tp1, tp2, NoApprox)
@@ -161,7 +165,8 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         try {
           pendingSubTypes += p
           firstTry
-        } finally {
+        }
+        finally {
           pendingSubTypes -= p
         }
       }

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

@@ -0,0 +1,158 @@
+package dotty.tools
+package dotc
+package core
+
+import util.common._
+import Types._
+import Symbols._
+import Flags._
+import Names._
+import Contexts._
+import SymDenotations._
+import Denotations._
+import Decorators._
+import reporting.diagnostic.Message
+import reporting.diagnostic.messages._
+import ast.untpd
+import config.Printers.cyclicErrors
+
+class TypeError(msg: String) extends Exception(msg) {
+  def this() = this("")
+  def toMessage(implicit ctx: Context): Message = getMessage
+}
+
+class MalformedType(pre: Type, denot: Denotation, absMembers: Set[Name]) extends TypeError {
+  override def toMessage(implicit ctx: Context): Message =
+    i"malformed type: $pre is not a legal prefix for $denot because it contains abstract type member${if (absMembers.size == 1) "" else "s"} ${absMembers.mkString(", ")}"
+}
+
+class MissingType(pre: Type, name: Name) extends TypeError {
+  private def otherReason(pre: Type)(implicit ctx: Context): String = pre match {
+    case pre: ThisType if pre.cls.givenSelfType.exists =>
+      i"\nor the self type of $pre might not contain all transitive dependencies"
+    case _ => ""
+  }
+
+  override def toMessage(implicit ctx: Context): Message = {
+    if (ctx.debug) printStackTrace()
+    i"""cannot resolve reference to type $pre.$name
+       |the classfile defining the type might be missing from the classpath${otherReason(pre)}"""
+  }
+}
+
+class RecursionOverflow(val op: String, details: => String, previous: Throwable, val weight: Int) extends TypeError {
+
+  def explanation = s"$op $details"
+
+  private def recursions: List[RecursionOverflow] = {
+    val nested = previous match {
+      case previous: RecursionOverflow => previous.recursions
+      case _ => Nil
+    }
+    this :: nested
+  }
+
+  def opsString(rs: List[RecursionOverflow])(implicit ctx: Context): String = {
+    val maxShown = 20
+    if (rs.lengthCompare(maxShown) > 0)
+      i"""${opsString(rs.take(maxShown / 2))}
+         |  ...
+         |${opsString(rs.takeRight(maxShown / 2))}"""
+    else
+      (rs.map(_.explanation): List[String]).mkString("\n  ", "\n|  ", "")
+  }
+
+  override def toMessage(implicit ctx: Context): Message = {
+    val mostCommon = recursions.groupBy(_.op).toList.maxBy(_._2.map(_.weight).sum)._2.reverse
+    s"""Recursion limit exceeded.
+       |Maybe there is an illegal cyclic reference?
+       |If that's not the case, you could also try to increase the stacksize using the -Xss JVM option.
+       |A recurring operation is (inner to outer):
+       |${opsString(mostCommon)}""".stripMargin
+  }
+
+  override def fillInStackTrace(): Throwable = this
+  override def getStackTrace() = previous.getStackTrace()
+}
+
+/** Post-process exceptions that might result from StackOverflow to add
+  * tracing information while unwalking the stack.
+  */
+// Beware: Since this object is only used when handling a StackOverflow, this code
+// cannot consume significant amounts of stack.
+object handleRecursive {
+  def apply(op: String, details: => String, exc: Throwable, weight: Int = 1)(implicit ctx: Context): Nothing = {
+    if (ctx.settings.YnoDecodeStacktraces.value) {
+      throw exc
+    } else {
+      exc match {
+        case _: RecursionOverflow =>
+          throw new RecursionOverflow(op, details, exc, weight)
+        case _ =>
+          var e = exc
+          while (e != null && !e.isInstanceOf[StackOverflowError]) e = e.getCause
+          if (e != null) throw new RecursionOverflow(op, details, e, weight)
+          else throw exc
+      }
+    }
+  }
+}
+
+class CyclicReference private (val denot: SymDenotation) extends TypeError {
+
+  override def toMessage(implicit ctx: Context) = {
+
+    def errorMsg(cx: Context): Message =
+      if (cx.mode is Mode.InferringReturnType) {
+        cx.tree match {
+          case tree: untpd.DefDef if !tree.tpt.typeOpt.exists =>
+            OverloadedOrRecursiveMethodNeedsResultType(tree.name)
+          case tree: untpd.ValDef if !tree.tpt.typeOpt.exists =>
+            RecursiveValueNeedsResultType(tree.name)
+          case _ =>
+            errorMsg(cx.outer)
+        }
+      }
+      else CyclicReferenceInvolving(denot)
+
+    val cycleSym = denot.symbol
+    if (cycleSym.is(Implicit, butNot = Method) && cycleSym.owner.isTerm)
+      CyclicReferenceInvolvingImplicit(cycleSym)
+    else
+      errorMsg(ctx)
+  }
+}
+
+object CyclicReference {
+  def apply(denot: SymDenotation)(implicit ctx: Context): CyclicReference = {
+    val ex = new CyclicReference(denot)
+    if (!(ctx.mode is Mode.CheckCyclic)) {
+      cyclicErrors.println(ex.getMessage)
+      for (elem <- ex.getStackTrace take 200)
+        cyclicErrors.println(elem.toString)
+    }
+    ex
+  }
+}
+
+class MergeError(val sym1: Symbol, val sym2: Symbol, val tp1: Type, val tp2: Type, prefix: Type) extends TypeError {
+
+  private def showSymbol(sym: Symbol)(implicit ctx: Context): String =
+    if (sym.exists) sym.showLocated else "[unknown]"
+
+  private def showType(tp: Type)(implicit ctx: Context) = tp match {
+    case ClassInfo(_, cls, _, _, _) => cls.showLocated
+    case _ => tp.show
+  }
+
+  protected def addendum(implicit ctx: Context) =
+    if (prefix `eq` NoPrefix) "" else i"\nas members of type $prefix"
+
+  override def toMessage(implicit ctx: Context): Message = {
+    if (ctx.debug) printStackTrace()
+    i"""cannot merge
+       |  ${showSymbol(sym1)} of type ${showType(tp1)}  and
+       |  ${showSymbol(sym2)} of type ${showType(tp2)}$addendum
+       """
+  }
+}

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

@@ -326,10 +326,4 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
 
 object TypeOps {
   @sharable var track = false // !!!DEBUG
-
-  /** When a property with this key is set in a context, it limits the number
-   *  of recursive member searches. If the limit is reached, findMember returns
-   *  NoDenotation.
-   */
-  val findMemberLimit = new Property.Key[Unit]
 }