Don't keep SkolemTypes in TASTY

SkolemTypes are no longer needed when Pickling because they exist only
for checking type-safety. After the typer, we can safely eliminate them.
Not having skolem types in TASTY simplifies the format and avoids having
to explain a difficult concept.

Author: odersky

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

@@ -104,7 +104,6 @@ Standard-Section: "ASTs" TopLevelStat*
                   TERMREF               possiblySigned_NameRef qual_Type
                   THIS                  clsRef_Type
                   REFINEDthis           refinedType_ASTRef
-                  SKOLEMtype            Type_ASTRef
                   SHARED                path_ASTRef
 
   Constant      = UNITconst
@@ -261,17 +260,16 @@ object TastyFormat {
   final val TYPEREFdirect = 66
   final val TERMREFpkg = 67
   final val TYPEREFpkg = 68
-  final val SKOLEMtype = 69
-  final val REFINEDthis = 70
-  final val BYTEconst = 71
-  final val SHORTconst = 72
-  final val CHARconst = 73
-  final val INTconst = 74
-  final val LONGconst = 75
-  final val FLOATconst = 76
-  final val DOUBLEconst = 77
-  final val STRINGconst = 78
-  final val IMPORTED = 79
+  final val REFINEDthis = 69
+  final val BYTEconst = 70
+  final val SHORTconst = 71
+  final val CHARconst = 72
+  final val INTconst = 73
+  final val LONGconst = 74
+  final val FLOATconst = 75
+  final val DOUBLEconst = 76
+  final val STRINGconst = 77
+  final val IMPORTED = 78
 
   final val THIS = 96
   final val CLASSconst = 97
@@ -421,7 +419,7 @@ object TastyFormat {
     case TYPEREFdirect => "TYPEREFdirect"
     case TERMREFpkg => "TERMREFpkg"
     case TYPEREFpkg => "TYPEREFpkg"
-    case SKOLEMtype => "SKOLEMtype"
+    case REFINEDthis => "REFINEDthis"
     case BYTEconst => "BYTEconst"
     case SHORTconst => "SHORTconst"
     case CHARconst => "CHARconst"

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

@@ -202,7 +202,6 @@ class TreePickler(pickler: TastyPickler) {
         writeByte(REFINEDthis)
         writeRef(pickledTypes.get(tpe.binder).asInstanceOf[Addr])
       case tpe: SkolemType =>
-        writeByte(SKOLEMtype)
         pickleType(tpe.info)
       case tpe: RefinedType =>
         val args = tpe.argInfos(interpolate = false)

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

@@ -256,8 +256,6 @@ class TreeUnpickler(reader: TastyReader, tastyName: TastyName.Table) {
           ThisType.raw(readType().asInstanceOf[TypeRef])
         case REFINEDthis =>
           RefinedThis(readTypeRef().asInstanceOf[RefinedType])
-        case SKOLEMtype =>
-          SkolemType(readTypeRef())
         case SHARED =>
           val ref = readAddr()
           typeAtAddr.getOrElseUpdate(ref, forkAt(ref).readType())

src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -231,7 +231,8 @@ class PlainPrinter(_ctx: Context) extends Printer {
       case tp: RefinedThis =>
         s"${nameString(tp.binder.typeSymbol)}{...}.this"
       case tp: SkolemType =>
-        "<unknown instance of type " ~ toTextGlobal(tp.info) ~ ">"
+        if (homogenizedView) toText(tp.info)
+        else "<unknown instance of type " ~ toTextGlobal(tp.info) ~ ">"
     }
   }
 

src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -179,7 +179,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
    *
    *  The body is simplified  as follows
    *   - if it is a TypeAlias, follow it
-   *   - replace all references to of the form <skolem>.HK$i by references
+   *   - replace all references to of the form <refined-this>.HK$i by references
    *     without a prefix, because the latter print nicer.
    *
    */