Fix #3816: Protect against cycles when unpickling hk types

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

@@ -5,6 +5,7 @@ package core
 import Periods._, Contexts._, Symbols._, Denotations._, Names._, NameOps._, Annotations._
 import Types._, Flags._, Decorators._, DenotTransformers._, StdNames._, Scopes._, Comments._
 import NameOps._, NameKinds._, Phases._
+import TypeApplications.TypeParamInfo
 import Scopes.Scope
 import collection.mutable
 import collection.BitSet
@@ -1915,6 +1916,11 @@ object SymDenotations {
       override def complete(denot: SymDenotation)(implicit ctx: Context) = self.complete(denot)
     }
 
+    /** The type parameters computed by the completer before completion has finished */
+    def completerTypeParams(sym: Symbol)(implicit ctx: Context): List[TypeParamInfo] =
+      if (sym is Touched) Nil // return `Nil` instead of throwing a cyclic reference
+      else sym.info.typeParams
+
     def decls: Scope = myDecls
     def sourceModule(implicit ctx: Context): Symbol = mySourceModuleFn(ctx)
     def moduleClass(implicit ctx: Context): Symbol = myModuleClassFn(ctx)
@@ -1924,12 +1930,12 @@ object SymDenotations {
     def withModuleClass(moduleClassFn: Context => Symbol): this.type = { myModuleClassFn = moduleClassFn; this }
   }
 
-  /** A subclass of LazyTypes where type parameters can be completed independently of
-   *  the info.
+  /** A subtrait of LazyTypes where completerTypeParams yields a List[TypeSymbol], which
+   *  should be completed independently of the info.
    */
   trait TypeParamsCompleter extends LazyType {
-    /** The type parameters computed by the completer before completion has finished */
-    def completerTypeParams(sym: Symbol)(implicit ctx: Context): List[TypeSymbol]
+    override def completerTypeParams(sym: Symbol)(implicit ctx: Context): List[TypeSymbol] =
+      unsupported("completerTypeParams") // should be abstract, but Scala-2 will then compute the wrong type for it
   }
 
   val NoSymbolFn = (ctx: Context) => NoSymbol

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

@@ -173,8 +173,10 @@ class TypeApplications(val self: Type) extends AnyVal {
         val tsym = self.symbol
         if (tsym.isClass) tsym.typeParams
         else if (!tsym.exists) self.info.typeParams
-        else if (!tsym.isCompleting) tsym.info.typeParams
-        else Nil
+        else tsym.infoOrCompleter match {
+          case info: LazyType => info.completerTypeParams(tsym)
+          case info => info.typeParams
+        }
       case self: AppliedType =>
         if (self.tycon.typeSymbol.isClass) Nil
         else self.superType.typeParams

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -745,12 +745,18 @@ class TreeUnpickler(reader: TastyReader,
             }
             TypeDef(readTemplate(localCtx))
           } else {
+            sym.info = TypeBounds.empty // needed to avoid cyclic references when unpicklin rhs, see i3816.scala
+            sym.setFlag(Provisional)
             val rhs = readTpt()(localCtx)
-            sym.info = NoCompleter
+            sym.info = new NoCompleter {
+              override def completerTypeParams(sym: Symbol)(implicit ctx: Context) =
+                rhs.tpe.typeParams
+            }
             sym.info = rhs.tpe match {
               case _: TypeBounds | _: ClassInfo => checkNonCyclic(sym, rhs.tpe, reportErrors = false)
               case _ => TypeAlias(rhs.tpe)
             }
+            sym.resetFlag(Provisional)
             TypeDef(rhs)
           }
         case PARAM =>

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

@@ -614,7 +614,7 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
     /** Force reading type params early, we need them in setClassInfo of subclasses. */
     def init()(implicit ctx: Context) = loadTypeParams
 
-    def completerTypeParams(sym: Symbol)(implicit ctx: Context): List[TypeSymbol] =
+    override def completerTypeParams(sym: Symbol)(implicit ctx: Context): List[TypeSymbol] =
       loadTypeParams
   }
 

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -239,7 +239,7 @@ object Checking {
           }
           if (isInteresting(pre)) {
             val pre1 = this(pre, false, false)
-            if (locked.contains(tp) || tp.symbol.infoOrCompleter == NoCompleter)
+            if (locked.contains(tp) || tp.symbol.infoOrCompleter.isInstanceOf[NoCompleter])
               throw CyclicReference(tp.symbol)
             locked += tp
             try checkInfo(tp.info)

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -824,7 +824,7 @@ class Namer { typer: Typer =>
     private[this] var nestedCtx: Context = null
     assert(!original.isClassDef)
 
-    def completerTypeParams(sym: Symbol)(implicit ctx: Context): List[TypeSymbol] = {
+    override def completerTypeParams(sym: Symbol)(implicit ctx: Context): List[TypeSymbol] = {
       if (myTypeParams == null) {
         //println(i"completing type params of $sym in ${sym.owner}")
         nestedCtx = localContext(sym).setNewScope
@@ -1229,6 +1229,7 @@ class Namer { typer: Typer =>
       sym.info = NoCompleter
       sym.info = checkNonCyclic(sym, unsafeInfo, reportErrors = true)
     }
+    sym.resetFlag(Provisional)
 
     // Here we pay the price for the cavalier setting info to TypeBounds.empty above.
     // We need to compensate by reloading the denotation of references that might

tests/pickling/i3816.scala

@@ -0,0 +1,4 @@
+trait Iterable { self =>
+  //type CC <: Iterable { type CC = self.CC }
+  type DD[X] <: Iterable { type DD[Y] = self.DD[Y] }
+}