Extend "cannot override mutable variable" restriction to deferred variables.

Extend "cannot override mutable variable" restriction also to deferred variables.
This aligns the behavior with Scala 2.

Fixes #14722

Author: odersky

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

@@ -474,7 +474,11 @@ object RefChecks {
           overrideError("needs `override` modifier")
       else if (other.is(AbsOverride) && other.isIncompleteIn(clazz) && !member.is(AbsOverride))
         overrideError("needs `abstract override` modifiers")
-      else if member.is(Override) && other.is(Accessor, butNot = Deferred) && other.accessedFieldOrGetter.is(Mutable, butNot = Lazy) then
+      else if member.is(Override)
+          && (other.is(Mutable)
+              || other.is(Accessor, butNot = Deferred)
+                  && other.accessedFieldOrGetter.is(Mutable, butNot = Lazy))
+      then
         overrideError("cannot override a mutable variable")
       else if (member.isAnyOverride &&
         !(member.owner.thisType.baseClasses exists (_ isSubClass other.owner)) &&

tests/neg/i14722.scala

@@ -0,0 +1,9 @@
+abstract class HasId(var id: String)
+
+case class Entity(override val id: String) extends HasId(id) // error
+
+object Test extends App {
+  val entity = Entity("0001")
+  entity.id = "0002"
+  println(entity.id)
+}