Do not store to non-existent fields in trait setters.

Author: sjrd

compiler/src/dotty/tools/dotc/core/NameOps.scala

@@ -259,9 +259,13 @@ object NameOps {
     def setterName: TermName = name.exclude(FieldName) ++ str.SETTER_SUFFIX
 
     def getterName: TermName =
-      name.exclude(FieldName).mapLast(n =>
-        if (n.endsWith(str.SETTER_SUFFIX)) n.take(n.length - str.SETTER_SUFFIX.length).asSimpleName
-        else n)
+      if name.is(TraitSetterName) then
+        val TraitSetterName(_, original) = name
+        original.getterName
+      else
+        name.exclude(FieldName).mapLast(n =>
+          if (n.endsWith(str.SETTER_SUFFIX)) n.take(n.length - str.SETTER_SUFFIX.length).asSimpleName
+          else n)
 
     def fieldName: TermName =
       if (name.isSetterName)

compiler/src/dotty/tools/dotc/transform/Memoize.scala

@@ -11,6 +11,7 @@ import SymUtils._
 import Constants._
 import ast.Trees._
 import MegaPhase._
+import NameKinds.TraitSetterName
 import NameOps._
 import Flags._
 import Decorators._
@@ -114,7 +115,8 @@ class Memoize extends MiniPhase with IdentityDenotTransformer { thisPhase =>
         EmptyTree
       }
 
-    if (sym.is(Accessor, butNot = NoFieldNeeded)) {
+    if (sym.is(Accessor, butNot = NoFieldNeeded)
+        && (!sym.name.is(TraitSetterName) || sym.getter.is(Accessor, butNot = NoFieldNeeded))) {
       val field = sym.field.orElse(newField).asTerm
 
       def adaptToField(tree: Tree): Tree =

tests/run/final-fields.check

@@ -2,6 +2,6 @@ T.f1
 T.f2
 T.f3
 T.f4
-3 2 0 0
+3 2 -3 -4
 3
 g