Add type argument encoding to SPLICEPATTERN

Future proof for #18271.

Author: nicolasstucki

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

@@ -725,10 +725,14 @@ class TreePickler(pickler: TastyPickler, attributes: Attributes) {
             bindings.foreach(pickleTree)
           }
         case SplicePattern(pat, args) =>
+          val targs = Nil // SplicePattern `targs` will be added with #18271
           writeByte(SPLICEPATTERN)
           withLength {
             pickleTree(pat)
             pickleType(tree.tpe)
+            for targ <- targs do
+              writeByte(EXPLICITtpt)
+              pickleTree(targ)
             args.foreach(pickleTree)
           }
         case Hole(_, idx, args, _) =>

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

@@ -1566,7 +1566,8 @@ class TreeUnpickler(reader: TastyReader,
             case SPLICEPATTERN =>
               val pat = readTree()
               val patType = readType()
-              val args = until(end)(readTree())
+              val (targs, args) = until(end)(readTree()).span(_.isType)
+              assert(targs.isEmpty, "unexpected type arguments in SPLICEPATTERN") // `targs` will be needed for #18271. Until this fearure is added they should be empty.
               SplicePattern(pat, args, patType)
             case HOLE =>
               readHole(end, isTerm = true)