Fix #4451: Set flag in the correct order

[Medowhill]

No description provided.

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[allanrenucci]

I would use an assertion here and remove the check file. Also since you will add tests for other type of lazy vals, you could do something like:

class InitError extends Exception

class Lazy(x: Int) {
  private[this] var init = false;
  lazy val value = {
    if (!init) {
      init = true
      throw new InitError
    }
    x
  }
}

class LazyVolatile(x: Int) {
  private[this] var init = false;
  @volatile lazy val value = {
    if (!init) {
      init = true
      throw new InitError
    }
    x
  }
}

def tryTwice(op: () => Int): Int =
  try { op(); assert(false) }
  catch { _: InitError => op() }

val l1 = new Lazy(42)
val l2 = new LazyVolatile(42)
assert(tryTwice(() => l1.value) == 42)
assert(tryTwice(() => l2.value) == 42)
...

[allanrenucci]

Can we add tests for lazy vals in methods as well as @volatile lazy vals

[allanrenucci]

I would rewrite as:

val stats = new mutable.ListBuffer[Tree]
if (!isWildcardArg(rhs)) stats += target.becomes(rhs)
stats += flag.becomes(Literal(Constant(true)))
stats += nullOut(nullables)
val init = Block(stats.toList, target.ensureApplied)

[allanrenucci]

LGTM. Can you squash your commits