Skolemize unstable prefixes in asSeenFrom

Skolemize unstable prefixes in asSeenFrom provided

 - the prefix appears at least once in non-variant or contra-variant position
 - we are in phase typer.

After typer, we have already established soundness, so there's no need to
do skolemization again. We can simply do the (otherwise unsound) substitution
from this-type to prefix.

Author: odersky

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

@@ -285,6 +285,9 @@ object Phases {
      */
     def relaxedTyping: Boolean = false
 
+    /** Overridden by FrontEnd */
+    def isTyper = false
+
     def exists: Boolean = true
 
     private var myPeriod: Period = Periods.InvalidPeriod

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

@@ -274,6 +274,11 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         case _ =>
           thirdTry(tp1, tp2)
       }
+    case tp1: SkolemType =>
+      tp2 match {
+        case tp2: SkolemType if !ctx.phase.isTyper && tp1.info <:< tp2.info => true
+        case _ => thirdTry(tp1, tp2)
+      }
     case tp1: TypeVar =>
       isSubType(tp1.underlying, tp2)
     case tp1: WildcardType =>

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

@@ -13,19 +13,19 @@ import ast.tpd._
 trait TypeOps { this: Context => // TODO: Make standalone object.
 
   final def asSeenFrom(tp: Type, pre: Type, cls: Symbol): Type = {
-    val m = if (pre.isStable || ctx.isAfterTyper) null else new AsSeenFromMap(pre, cls)
-    asSeenFrom(tp, pre, cls, null)
+    val m = if (pre.isStable || !ctx.phase.isTyper) null else new AsSeenFromMap(pre, cls)
+    var res = asSeenFrom(tp, pre, cls, m)
+    if (m != null && m.unstable) asSeenFrom(tp, SkolemType(pre), cls) else res
   }
-    
+
   final def asSeenFrom(tp: Type, pre: Type, cls: Symbol, theMap: AsSeenFromMap): Type = {
 
     def toPrefix(pre: Type, cls: Symbol, thiscls: ClassSymbol): Type = /*>|>*/ ctx.conditionalTraceIndented(TypeOps.track, s"toPrefix($pre, $cls, $thiscls)") /*<|<*/ {
       if ((pre eq NoType) || (pre eq NoPrefix) || (cls is PackageClass))
         tp
       else if (thiscls.derivesFrom(cls) && pre.baseTypeRef(thiscls).exists) {
-        if (!pre.isStable && theMap != null && theMap.currentVariance <= 0) {
+        if (theMap != null && theMap.currentVariance <= 0 && !pre.isStable)
           theMap.unstable = true
-        }
         pre match {
           case SuperType(thispre, _) => thispre
           case _ => pre
@@ -43,10 +43,13 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
           val sym = tp.symbol
           if (sym.isStatic) tp
           else {
+            val prevStable = theMap == null || !theMap.unstable
             val pre1 = asSeenFrom(tp.prefix, pre, cls, theMap)
-            if (theMap != null && theMap.unstable) {
+            if (theMap != null && theMap.unstable && prevStable) {
               pre1.member(tp.name).info match {
-                case TypeAlias(alias) => return alias
+                case TypeAlias(alias) =>
+                  theMap.unstable = false
+                  return alias
                 case _ =>
               }
             }
@@ -61,7 +64,7 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
             asSeenFrom(tp.parent, pre, cls, theMap),
             tp.refinedName,
             asSeenFrom(tp.refinedInfo, pre, cls, theMap))
-        case tp: TypeAlias  =>
+        case tp: TypeAlias if theMap == null => // if theMap exists, need to do the variance calculation
           tp.derivedTypeAlias(asSeenFrom(tp.alias, pre, cls, theMap))
         case _ =>
           (if (theMap != null) theMap else new AsSeenFromMap(pre, cls))

src/dotty/tools/dotc/typer/FrontEnd.scala

@@ -13,6 +13,7 @@ import scala.util.control.NonFatal
 class FrontEnd extends Phase {
 
   override def phaseName = "frontend"
+  override def isTyper = true
 
   def monitor(doing: String)(body: => Unit)(implicit ctx: Context) =
     try body

test/dotc/tests.scala

@@ -133,15 +133,14 @@ class tests extends CompilerTest {
   @Test def neg_i0091_infpaths = compileFile(negDir, "i0091-infpaths", xerrors = 3)
   @Test def neg_i0248_inherit_refined = compileFile(negDir, "i0248-inherit-refined", xerrors = 4)
   @Test def neg_i0281 = compileFile(negDir, "i0281-null-primitive-conforms", xerrors = 3)
+  @Test def neg_i583 = compileFile(negDir, "i0583-skolemize", xerrors = 2)
   @Test def neg_moduleSubtyping = compileFile(negDir, "moduleSubtyping", xerrors = 4)
   @Test def neg_escapingRefs = compileFile(negDir, "escapingRefs", xerrors = 2)
   @Test def neg_instantiateAbstract = compileFile(negDir, "instantiateAbstract", xerrors = 8)
   @Test def neg_selfInheritance = compileFile(negDir, "selfInheritance", xerrors = 5)
 
-
   @Test def run_all = runFiles(runDir)
 
-
   @Test def dotty = compileDir(dottyDir, "tools", "-deep" :: allowDeepSubtypes ++ twice) // note the -deep argument
 
 

tests/neg/i0583-skolemize.scala

@@ -0,0 +1,27 @@
+import scala.collection.mutable.ListBuffer
+
+object Test1 {
+
+  class Box[T](x: T)
+  def box[T](x: T) = new Box[T](x)
+
+  class C[T] { def x: T = ???; def x_=(y: T): Unit = ??? }
+
+  val c: C[Int] = ???
+  val d: C[Float] = ???
+
+  val xs: List[C[_]] = List(c, d)
+
+  xs(0).x = xs(1).x
+
+}
+object Test {
+  def main(args: Array[String]): Unit = {
+    val f: ListBuffer[Int] = ListBuffer(1,2)
+    val g: ListBuffer[Double] = ListBuffer(3.0,4.0)
+    val lb: ListBuffer[ListBuffer[_]] = ListBuffer(f, g)
+    lb(0)(0) = lb(1)(0)
+    val x: Int = f(0)
+  }
+}
+