[Analyzer] Nullsafety on final after if, gives an error

[ikbendewilliam]

Small issue with the analyser regarding to nullsafety
I'm using version Dart SDK version: 2.14.2 (stable) (Wed Sep 15 12:32:06 2021 +0200) on "macos_x64"

We have the following flutter code. _controller is a nullable InAppWebViewController?. To not have to include a ! when we use it, we add a final final controller = _controller and do a check await controller?.canGoBack() == true. In this case, controller is never null, since controller?.anything will return null and null != true 😁. Therefore await controller.goBack(); should be fine, but it throws an error.
The method 'goBack' can't be unconditionally invoked because the receiver can be 'null'. Try making the call conditional (using '?.') or adding a null check to the target ('!').dart(unchecked_use_of_nullable_value)

  Widget build(BuildContext context) {
    final controller = _controller;
      return WillPopScope(
        onWillPop: () async {
          if (await controller?.canGoBack() == true) {
            await controller.goBack();
            return false;
          }
          return true;
        },

if we instead do

if (controller != null && await controller.canGoBack() == true) {
  await controller.goBack();
...

no errors are thrown. And everything compiles.
If I simplify the code further, this becomes more apparent:

onBackClicked: () async {
  final controller = _controller;
  if (controller?.canGoBack == true) {
    controller.goBack(); <-- The method 'goBack' can't be unconditionally invoked because the receiver can be 'null'.
    return;
  }

Is there any way that controller can be null?
Otherwise, I would suggest to check if the analyser can detect this and allow us to not have to check for null again.

[lrhn]

Dart promotes the types of variables when you do certain checks on the variable itself.

Here the controller != null check will promote the controller variable to a non-null type along the true branch.

The check controller?.canGoBack == true does not check the variable controller directly, so there is no promotion.
It's logically correct that if controller?.canGoBack is the non-null value true, then controller cannot be null, but that's more logical inference steps than the compiler will do for you.

We are aware of this short-coming (see dart-lang/language#1224). We haven't decided whether to increase the complexity of the non-null inference to support this case yet.

[ikbendewilliam]

Oke, thank you for your response