Fix #1878: Generate fields for final vars.

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

@@ -102,7 +102,7 @@ import Decorators._
           case _ => t
         }
         skipBlocks(tree.rhs) match {
-          case lit: Literal if sym.is(Final) && isIdempotentExpr(tree.rhs) =>
+          case lit: Literal if sym.is(Final, butNot = Mutable) && isIdempotentExpr(tree.rhs) =>
             // duplicating scalac behavior: for final vals that have rhs as constant, we do not create a field
             // and instead return the value. This seemingly minor optimization has huge effect on initialization
             // order and the values that can be observed during superconstructor call

tests/run/final-var.check

@@ -0,0 +1,4 @@
+false
+true
+false
+true

tests/run/final-var.scala

@@ -0,0 +1,20 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    println(Obj.myFinalVar)
+    Obj.myFinalVar = true
+    println(Obj.myFinalVar)
+
+    val o = new Cls
+    println(o.myFinalVar)
+    o.myFinalVar = true
+    println(o.myFinalVar)
+  }
+}
+
+object Obj {
+  final var myFinalVar: Boolean = false
+}
+
+class Cls {
+  final var myFinalVar: Boolean = false
+}