Fix positions in assignments

TreeCopier#finalize will set the position of the new tree to the old
tree, which is not what we want here since the desugared tree position
should contain more than the position of the parsed assignment lhs.
Moreover, the desugared tree will never be identical to the parsed lhs
so there is no reason to use a TreeCopier at all.

Author: smarter

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -567,13 +567,14 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
   def typedAssign(tree: untpd.Assign, pt: Type)(implicit ctx: Context) = track("typedAssign") {
     tree.lhs match {
       case lhs @ Apply(fn, args) =>
-        typed(cpy.Apply(lhs)(untpd.Select(fn, nme.update), args :+ tree.rhs), pt)
+        typed(untpd.Apply(untpd.Select(fn, nme.update), args :+ tree.rhs), pt)
       case untpd.TypedSplice(Apply(MaybePoly(Select(fn, app), targs), args)) if app == nme.apply =>
         val rawUpdate: untpd.Tree = untpd.Select(untpd.TypedSplice(fn), nme.update)
         val wrappedUpdate =
           if (targs.isEmpty) rawUpdate
           else untpd.TypeApply(rawUpdate, targs map (untpd.TypedSplice(_)))
-        val appliedUpdate = cpy.Apply(fn)(wrappedUpdate, (args map (untpd.TypedSplice(_))) :+ tree.rhs)
+        val appliedUpdate =
+          untpd.Apply(wrappedUpdate, (args map (untpd.TypedSplice(_))) :+ tree.rhs)
         typed(appliedUpdate, pt)
       case lhs =>
         val lhsCore = typedUnadapted(lhs, AssignProto)
@@ -604,7 +605,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
                   val setterType = ensureAccessible(setterTypeRaw, isSuperSelection(lhsCore), tree.pos)
                   val lhs2 = healNonvariant(
                     untpd.rename(lhsCore, setterName).withType(setterType), WildcardType)
-                  typedUnadapted(cpy.Apply(tree)(untpd.TypedSplice(lhs2), tree.rhs :: Nil))
+                  typedUnadapted(untpd.Apply(untpd.TypedSplice(lhs2), tree.rhs :: Nil))
                 case _ =>
                   reassignmentToVal
               }