Fix #4451: Set flag in the correct order

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

@@ -218,11 +218,10 @@ class LazyVals extends MiniPhase with IdentityDenotTransformer {
     * ```
     */
   def mkNonThreadSafeDef(target: Tree, flag: Tree, rhs: Tree, nullables: List[Symbol])(implicit ctx: Context) = {
-    val setFlag = flag.becomes(Literal(Constants.Constant(true)))
-    val setNullables = nullOut(nullables)
-    val setTargetAndNullable = if (isWildcardArg(rhs)) setNullables else target.becomes(rhs) :: setNullables
-    val init = Block(setFlag :: setTargetAndNullable, target.ensureApplied)
-    If(flag.ensureApplied, target.ensureApplied, init)
+    val stats = new mutable.ListBuffer[Tree]
+    if (!isWildcardArg(rhs)) stats += target.becomes(rhs)
+    stats += flag.becomes(Literal(Constants.Constant(true))) ++= nullOut(nullables)
+    If(flag.ensureApplied, target.ensureApplied, Block(stats.toList, target.ensureApplied))
   }
 
   /** Create non-threadsafe lazy accessor for not-nullable types  equivalent to such code

tests/run/i4451.scala

@@ -0,0 +1,55 @@
+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
+  }
+}
+
+object Test {
+
+  def tryTwice(x: => Int): Int =
+    try {
+      x; assert(false); 0
+    } catch {
+      case _: InitError => x
+    }
+
+  def lazyInMethod(x: Int) = {
+    var init = false
+    lazy val value = {
+      if (!init) {
+        init = true
+        throw new InitError
+      }
+      x
+    }
+    tryTwice(value)
+  }
+
+  def main(args: Array[String]) = {
+    val v = 42
+
+    val l0 = new Lazy(v)
+    val l1 = new LazyVolatile(v)
+    assert(tryTwice(l0.value) == v)
+    assert(tryTwice(l1.value) == v)
+    assert(lazyInMethod(v) == v)
+  }
+}