Mark single-use field MUTABLE for nullification (Fixes scala/bug#11781)

Author: leitoh0

src/compiler/scala/tools/nsc/transform/Mixin.scala

@@ -409,8 +409,10 @@ abstract class Mixin extends InfoTransform with ast.TreeDSL with AccessorSynthes
             deriveDefDef(dd) {
               case blk@Block(stats, expr) =>
                 assert(dd.symbol.originalOwner.isClass, dd.symbol)
-                def nullify(sym: Symbol) =
+                def nullify(sym: Symbol) = {
+                  sym.accessedOrSelf.setFlag(MUTABLE)
                   Select(gen.mkAttributedThis(sym.enclClass), sym.accessedOrSelf) === NULL
+                }
                 val stats1 = stats ::: fieldsToNull.map(nullify)
                 treeCopy.Block(blk, stats1, expr)
               case tree =>

test/junit/scala/tools/nsc/backend/jvm/BytecodeTest.scala

@@ -303,4 +303,13 @@ class BytecodeTest extends BytecodeTesting {
     }
     assertEquals(List("$outer", "x$1", "y$1"), assignedInConstr.sorted)
   }
+
+  @Test
+  def t11718(): Unit = {
+    val code = "class A11718; class B11718 { private val a = new A; lazy val b = a }"
+    val cs = compileClasses(code)
+    val B = cs.find(_.name == "B11718").get
+    val a = B.fields.asScala.find(_.name == "a").get
+    assertEquals(0, a.access & Opcodes.ACC_FINAL)
+  }
 }