2.13 and JDK 11 - IllegalAccessError: Update to non-static final field

[pvlugter]

Uncovered by akka/akka#32128 when dropping JDK 8 in Akka (akka/akka#32127).

Additional case for scala/scala-dev#408 and #12340.

Trait fields are still marked final when implemented by an anonymous class in a lazy val.

Reproduction steps

scalaVersion := "2.13.12"
scalacOptions ++= Seq("-release", "11")

trait T {
  val foo = "foo"
}

object Test {
  def main(args: Array[String]): Unit = {
    lazy val t = new T {}
    println(t)
  }
}

Problem

java.lang.IllegalAccessError: Update to non-static final field Test$$anon$1.foo attempted from a different method (T$_setter_$foo_$eq) than the initializer method <init>

Field marked final:

:javap -p -v Test$$anon$1

...

  private final java.lang.String foo;
    descriptor: Ljava/lang/String;
    flags: (0x0012) ACC_PRIVATE, ACC_FINAL

When it's either not a lazy val or not an anonymous class, the field is non-final:

  private java.lang.String foo;
    descriptor: Ljava/lang/String;
    flags: (0x0002) ACC_PRIVATE

[lrytz]

This was a hard one to pin down...

Example

trait T { val foo = "foo" }
class C {
  def m = {
    lazy val t = new T {}
    /* expands to
    lazy val t = {
      class anon extends T {
        private var _foo
        def foo = _foo
        def T$_setter_$foo_= (s: String) = { _foo = s}
      }
      new anon
    }
    */
    t
  }
}

Fields info transformer:

• adds field symbol to class for field inherited from a trait
• the field symbol does not have the MTUABLE flag

Fields tree transformer:

• calls afterOwnPhase(clazz.info.decls) on the anonymous class, this forces the fields info transform
  • the type history has a new entry tailcalls -> ClassInfoType(...) (tailcalls is after fields)
• tree transformer adds the MUTABLE flag to the field symbol when generating the setter
• tree transformer creates the t$lzycompute accessor and calls typer on it
  • accessor body: t$lzy.synchronized { if (t$lzy.initialized) t$lzy.value else { t$lzy.initialize({ <anon> }}
  • typedApply of synchronized(...) -> inferMethodInstance -> TreeTypeSubstituter for type parameter of synchronized
  • TypeMapTreeSubstituter (parent of TreeTypeSubstituter) calls if (tree.isDef) tree.symbol modifyInfo typeMap
  • modifyInfo on the anonymous class symbol is destructive for the type history:
    => setInfo(f(info)) takes the current info and creates a new TypeHistory with just one element
    => the type history has the entry fields -> ClassInfoType(...) (the type itself is unchanged)

When obtaining the anonymous class info in a later phase, the fields info transformer will run again because the current type is only valid until fields. A new foo field symbol is created, it doesn't have the MUTABLE flag, so it ends up final in bytecode.