Fix erasure of this types and refactor value class erasure

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

@@ -32,8 +32,16 @@ object Config {
    */
   final val checkConstraintsPropagated = false
 
-  /** Check that constraints of globally committable typer states are closed */
-  final val checkConstraintsClosed = true
+  /** Check that constraints of globally committable typer states are closed.
+   *  NOTE: When enabled, the check can cause CyclicReference errors because
+   *  it traverses all elements of a type. Such failures were observed when
+   *  compiling all of dotty together (source seems to be in GenBCode which
+   *  accesses javac's settings.)
+   *
+   *  It is recommended to turn this option on only when chasing down
+   *  a PolyParam instantiation error. See comment in Types.TypeVar.instantiate.
+   */
+  final val debugCheckConstraintsClosed = false
 
   /** Check that no type appearing as the info of a SymDenotation contains
    *  skolem types.

src/dotty/tools/dotc/core/TypeErasure.scala

@@ -110,15 +110,20 @@ object TypeErasure {
   private def erasureCtx(implicit ctx: Context) =
     if (ctx.erasedTypes) ctx.withPhase(ctx.erasurePhase).addMode(Mode.FutureDefsOK) else ctx
 
-  /** The standard erasure of a Scala type.
+  /** The standard erasure of a Scala type. Value classes are erased as normal classes.
+   *
+   *  @param tp            The type to erase.
+  */
+  def erasure(tp: Type)(implicit ctx: Context): Type =
+    erasureFn(isJava = false, semiEraseVCs = false, isConstructor = false, wildcardOK = false)(tp)(erasureCtx)
+
+  /** The value class erasure of a Scala type, where value classes are semi-erased to
+   *  ErasedValueType (they will be fully erased in [[ElimErasedValueType]]).
    *
    *  @param tp            The type to erase.
-   *  @param semiEraseVCs  If true, value classes are semi-erased to ErasedValueType
-   *                       (they will be fully erased in [[ElimErasedValueType]]).
-   *                       If false, they are erased like normal classes.
    */
-  def erasure(tp: Type, semiEraseVCs: Boolean = false)(implicit ctx: Context): Type =
-    erasureFn(isJava = false, semiEraseVCs, isConstructor = false, wildcardOK = false)(tp)(erasureCtx)
+  def valueErasure(tp: Type)(implicit ctx: Context): Type =
+    erasureFn(isJava = false, semiEraseVCs = true, isConstructor = false, wildcardOK = false)(tp)(erasureCtx)
 
   def sigName(tp: Type, isJava: Boolean)(implicit ctx: Context): TypeName = {
     val seqClass = if (isJava) defn.ArrayClass else defn.SeqClass
@@ -141,7 +146,7 @@ object TypeErasure {
     case tp: ThisType =>
       tp
     case tp =>
-      erasure(tp, semiEraseVCs = true)
+      valueErasure(tp)
   }
 
   /**  The symbol's erased info. This is the type's erasure, except for the following symbols:
@@ -315,12 +320,8 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
       val parent = tp.parent
       if (parent isRef defn.ArrayClass) eraseArray(tp)
       else this(parent)
-    case tp: TermRef =>
+    case _: TermRef | _: ThisType =>
       this(tp.widen)
-    case tp: ThisType =>
-      def thisTypeErasure(tpToErase: Type) =
-        erasureFn(isJava, semiEraseVCs = false, isConstructor, wildcardOK)(tpToErase)
-      thisTypeErasure(tp.cls.typeRef)
     case SuperType(thistpe, supertpe) =>
       SuperType(this(thistpe), this(supertpe))
     case ExprType(rt) =>
@@ -396,7 +397,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
   private def eraseDerivedValueClassRef(tref: TypeRef)(implicit ctx: Context): Type = {
     val cls = tref.symbol.asClass
     val underlying = underlyingOfValueClass(cls)
-    ErasedValueType(cls, erasure(underlying, semiEraseVCs = true))
+    ErasedValueType(cls, valueErasure(underlying))
   }
 
 

src/dotty/tools/dotc/core/TyperState.scala

@@ -87,7 +87,7 @@ extends TyperState(r) {
 
   override def constraint = myConstraint
   override def constraint_=(c: Constraint)(implicit ctx: Context) = {
-    if (Config.checkConstraintsClosed && isGlobalCommittable) c.checkClosed()
+    if (Config.debugCheckConstraintsClosed && isGlobalCommittable) c.checkClosed()
     myConstraint = c
   }
 

src/dotty/tools/dotc/core/Types.scala

@@ -2460,6 +2460,8 @@ object Types {
 
       if (ctx.typerState.isGlobalCommittable)
         assert(!inst.isInstanceOf[PolyParam], i"bad inst $this := $inst, constr = ${ctx.typerState.constraint}")
+          // If this fails, you might want to turn on Config.debugCheckConstraintsClosed
+          // to help find the root of the problem.
 
       instantiateWith(inst)
     }

src/dotty/tools/dotc/transform/Erasure.scala

@@ -268,35 +268,29 @@ object Erasure extends TypeTestsCasts{
   class Typer extends typer.ReTyper with NoChecking {
     import Boxing._
 
-    def erasedType(tree: untpd.Tree, semiEraseVCs: Boolean)(implicit ctx: Context): Type =
-      tree.typeOpt match {
-        case tp: TermRef if tree.isTerm => erasedRef(tp)
-        case tp => erasure(tp, semiEraseVCs)
-      }
+    def erasedType(tree: untpd.Tree)(implicit ctx: Context): Type = {
+      val tp = tree.typeOpt
+      if (tree.isTerm) erasedRef(tp) else valueErasure(tp)
+    }
 
-    def promote(tree: untpd.Tree, semiEraseVCs: Boolean)(implicit ctx: Context): tree.ThisTree[Type] = {
+    override def promote(tree: untpd.Tree)(implicit ctx: Context): tree.ThisTree[Type] = {
       assert(tree.hasType)
-      val erased = erasedType(tree, semiEraseVCs)
+      val erased = erasedType(tree)
       ctx.log(s"promoting ${tree.show}: ${erased.showWithUnderlying()}")
       tree.withType(erased)
     }
 
-    override def promote(tree: untpd.Tree)(implicit ctx: Context): tree.ThisTree[Type] = {
-      promote(tree, true)
-    }
-
     /** When erasing most TypeTrees we should not semi-erase value types.
      *  This is not the case for [[DefDef#tpt]], [[ValDef#tpt]] and [[Typed#tpt]], they
      *  are handled separately by [[typedDefDef]], [[typedValDef]] and [[typedTyped]].
      */
-    override def typedTypeTree(tree: untpd.TypeTree, pt: Type)(implicit ctx: Context): TypeTree = {
-      promote(tree, semiEraseVCs = false)
-    }
+    override def typedTypeTree(tree: untpd.TypeTree, pt: Type)(implicit ctx: Context): TypeTree =
+      tree.withType(erasure(tree.tpe))
 
     /** This override is only needed to semi-erase type ascriptions */
     override def typedTyped(tree: untpd.Typed, pt: Type)(implicit ctx: Context): Tree = {
       val Typed(expr, tpt) = tree
-      val tpt1 = promote(tpt, semiEraseVCs = true)
+      val tpt1 = promote(tpt)
       val expr1 = typed(expr, tpt1.tpe)
       assignType(untpd.cpy.Typed(tree)(expr1, tpt1), tpt1)
     }
@@ -384,7 +378,7 @@ object Erasure extends TypeTestsCasts{
     }
 
     override def typedSelectFromTypeTree(tree: untpd.SelectFromTypeTree, pt: Type)(implicit ctx: Context) =
-      untpd.Ident(tree.name).withPos(tree.pos).withType(erasedType(tree, semiEraseVCs = false))
+      untpd.Ident(tree.name).withPos(tree.pos).withType(erasedType(tree))
 
     override def typedThis(tree: untpd.This)(implicit ctx: Context): Tree =
       if (tree.symbol == ctx.owner.enclosingClass || tree.symbol.isStaticOwner) promote(tree)
@@ -460,7 +454,7 @@ object Erasure extends TypeTestsCasts{
       if (pt.isValueType) pt else {
         if (tree.typeOpt.derivesFrom(ctx.definitions.UnitClass))
           tree.typeOpt
-        else erasure(tree.typeOpt, semiEraseVCs = true)
+        else valueErasure(tree.typeOpt)
       }
     }
 

src/dotty/tools/dotc/transform/ExtensionMethods.scala

@@ -14,7 +14,7 @@ import core._
 import Phases.Phase
 import Types._, Contexts._, Constants._, Names._, NameOps._, Flags._, DenotTransformers._
 import SymDenotations._, Symbols._, StdNames._, Annotations._, Trees._, Scopes._, Denotations._
-import TypeErasure.{ erasure, ErasedValueType }
+import TypeErasure.{ erasure, valueErasure, ErasedValueType }
 import TypeUtils._
 import util.Positions._
 import Decorators._
@@ -65,7 +65,7 @@ class ExtensionMethods extends MiniPhaseTransform with DenotTransformer with Ful
               }
             }
 
-            val underlying = erasure(underlyingOfValueClass(valueClass), semiEraseVCs = true)
+            val underlying = valueErasure(underlyingOfValueClass(valueClass))
             val evt = ErasedValueType(valueClass, underlying)
             val u2evtSym = ctx.newSymbol(moduleSym, nme.U2EVT, Synthetic | Method,
               MethodType(List(nme.x_0), List(underlying), evt))