Fixes to pickling and unpickling untyped trees

Among others, adds new tag for EMPTYTREE. The serialized info is ambiguous
otherwise. E.g. TypeBounds(Empty, B) and TypeBounds(B, Empty) serialize in
the same way.

Author: odersky

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

@@ -311,6 +311,7 @@ object TastyFormat {
   final val MACRO = 34
   final val ERASED = 35
   final val PARAMsetter = 36
+  final val EMPTYTREE = 37
 
   // Cat. 2:    tag Nat
 
@@ -445,7 +446,7 @@ object TastyFormat {
 
   /** Useful for debugging */
   def isLegalTag(tag: Int) =
-    firstSimpleTreeTag <= tag && tag <= PARAMsetter ||
+    firstSimpleTreeTag <= tag && tag <= EMPTYTREE ||
     firstNatTreeTag <= tag && tag <= SYMBOLconst ||
     firstASTTreeTag <= tag && tag <= SINGLETONtpt ||
     firstNatASTTreeTag <= tag && tag <= NAMEDARG ||
@@ -542,6 +543,7 @@ object TastyFormat {
     case DEFAULTparameterized => "DEFAULTparameterized"
     case STABLE => "STABLE"
     case PARAMsetter => "PARAMsetter"
+    case EMPTYTREE => "EMPTYTREE"
 
     case SHAREDterm => "SHAREDterm"
     case SHAREDtype => "SHAREDtype"

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

@@ -859,7 +859,8 @@ class TreePickler(pickler: TastyPickler) {
         writeByte(TYPEDSPLICE)
         withLength { pickleTree(splice) }
       case Thicket(trees) =>
-        trees.foreach(pickleUntyped)
+        if (trees.isEmpty) writeByte(EMPTYTREE)
+        else trees.foreach(pickleUntyped)
       case _ =>
         pickleUntyped(desugar(tree))
     }

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

@@ -1203,7 +1203,11 @@ class TreeUnpickler(reader: TastyReader,
       def readIdent(): untpd.Ident = readUntyped().asInstanceOf[untpd.Ident]
 
       def readParams[T <: untpd.MemberDef](tag: Int): List[T] =
-        collectWhile(nextByte == tag)(readUntyped().asInstanceOf[T])
+        collectWhile(nextByte == tag) {
+          import untpd.modsDeco
+          val m: T = readUntyped().asInstanceOf[T]
+          m.withMods(m.mods | Param).asInstanceOf[T]
+        }
 
       def readParamss(): List[List[untpd.ValDef]] =
         collectWhile(nextByte == PARAMS) {
@@ -1246,8 +1250,11 @@ class TreeUnpickler(reader: TastyReader,
           untpd.ByNameTypeTree(readUntyped())
         case NAMEDARG =>
           untpd.NamedArg(readName(), readUntyped())
+        case EMPTYTREE =>
+          untpd.EmptyTree
         case SHAREDtype =>
-          assert(readNat() == 0)
+          val b = readNat()
+          assert(b == 0, i"bad shared type $b at $currentAddr when reading ${ctx.owner.ownersIterator.toList}%, %")
           untpd.TypeTree()
         case _ =>
           untpd.Literal(readConstant(tag))
@@ -1353,7 +1360,11 @@ class TreeUnpickler(reader: TastyReader,
             untpd.TypedSplice(readTerm())
           case FUNCTION =>
             val body = readUntyped()
-            val params = until(end)(readUntyped())
+            import untpd.modsDeco
+            val params = until(end)(readUntyped()).map {
+              case param: untpd.ValDef => param.withMods(param.mods | Param)
+              case param => param
+            }
             untpd.Function(params, body)
           case INFIXOP =>
             untpd.InfixOp(readUntyped(), readIdent(), readUntyped())