Fix #2943: Insert LazyRefs automatically upon Unpickling

This relieves Tasty producers from having to insert LazyRefs
themselves. Fixes #2943.

Author: odersky

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

@@ -164,7 +164,6 @@ Standard-Section: "ASTs" TopLevelStat*
                   BIND           Length boundName_NameRef bounds_Type
                                         // for type-variables defined in a type pattern
                   BYNAMEtype            underlying_Type
-                  LAZYref               underlying_Type
                   POLYtype       Length result_Type NamesTypes
                   METHODtype     Length result_Type NamesTypes      // needed for refinements
                   TYPELAMBDAtype Length result_Type NamesTypes      // variance encoded in front of name: +/-/(nothing)
@@ -323,7 +322,6 @@ object TastyFormat {
   final val PROTECTEDqualified = 105
   final val RECtype = 106
   final val SINGLETONtpt = 107
-  final val LAZYref = 108
 
   final val IDENT = 112
   final val IDENTtpt = 113
@@ -514,7 +512,6 @@ object TastyFormat {
     case DOUBLEconst => "DOUBLEconst"
     case STRINGconst => "STRINGconst"
     case RECtype => "RECtype"
-    case LAZYref => "LAZYref"
 
     case IDENT => "IDENT"
     case IDENTtpt => "IDENTtpt"

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

@@ -246,7 +246,6 @@ class TreePickler(pickler: TastyPickler) {
     case tpe: ParamRef =>
       assert(pickleParamRef(tpe), s"orphan parameter reference: $tpe")
     case tpe: LazyRef =>
-      writeByte(LAZYref)
       pickleType(tpe.ref)
   }
 

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

@@ -6,6 +6,7 @@ package tasty
 import Contexts._, Symbols._, Types._, Scopes._, SymDenotations._, Names._, NameOps._
 import StdNames._, Denotations._, Flags._, Constants._, Annotations._
 import NameKinds._
+import typer.Checking.checkNonCyclic
 import util.Positions._
 import ast.{tpd, Trees, untpd}
 import Trees._
@@ -293,10 +294,6 @@ class TreeUnpickler(reader: TastyReader, nameAtRef: NameRef => TermName, posUnpi
           RecType(rt => registeringType(rt, readType()))
         case RECthis =>
           RecThis(readTypeRef().asInstanceOf[RecType])
-        case LAZYref =>
-          val rdr = fork
-          skipTree()
-          LazyRef(implicit ctx => rdr.readType())
         case SHARED =>
           val ref = readAddr()
           typeAtAddr.getOrElseUpdate(ref, forkAt(ref).readType())
@@ -678,8 +675,9 @@ class TreeUnpickler(reader: TastyReader, nameAtRef: NameRef => TermName, posUnpi
             TypeDef(readTemplate(localCtx))
           } else {
             val rhs = readTpt()
+            sym.info = NoCompleter
             sym.info = rhs.tpe match {
-              case _: TypeBounds | _: ClassInfo => rhs.tpe
+              case _: TypeBounds | _: ClassInfo => checkNonCyclic(sym, rhs.tpe, reportErrors = false)
               case _ => TypeAlias(rhs.tpe, sym.variance)
             }
             TypeDef(rhs)

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

@@ -239,7 +239,8 @@ object Checking {
           }
           if (isInteresting(pre)) {
             val pre1 = this(pre, false, false)
-            if (locked.contains(tp)) throw CyclicReference(tp.symbol)
+            if (locked.contains(tp) || tp.symbol.infoOrCompleter == NoCompleter)
+              throw CyclicReference(tp.symbol)
             locked += tp
             try checkInfo(tp.info)
             finally locked -= tp