Need null check after ??=

[Lysmi]

If I use "??=" dart analyzer don't understand that variable can not be null.
Example:

static void regCommand(String command, Function action, {bool? isMustRemove}) {
      teleDart.onCommand(command).listen((message) {
          isMustRemove ??= false;
          if (isMustRemove) {
              teleDart.deleteMessage(message.chat.id, message.message_id);
          }
    });
  }

This code have error:

But in reality isMustRemove can not be null. Of course, if I use {bool isMustRemove = false} is solve this case, but we have more cases with analyze null.
And if I write:

static void regCommand(String command, Function action, {bool? isMustRemove}) {
      teleDart.onCommand(command).listen((message) {
          isMustRemove ??= false;
          if(isMustRemove != null) {
              if (isMustRemove) {
                  teleDart.deleteMessage(message.chat.id, message.message_id);
              }
          }
    });
  }

or

static void regCommand(String command, Function action, {bool? isMustRemove}) {
      teleDart.onCommand(command).listen((message) {
          isMustRemove ??= false;
          if (isMustRemove as bool) {
              teleDart.deleteMessage(message.chat.id, message.message_id);
          }
    });
  }

all work.

[lrhn]

This is a limit of the Dart type inference algorithm.

The variable is declared in another function than where it's assigned. That's bad for inference because the function doing the assignment can escape, and that means that any other computation can potentially perform an assignment.

In this particular case, that can't happen because there is only one assignment, ever, but the inference algorithm is one-pass, so it doesn't go back and report that nothing bad can happen, it just gives up when it sees the captured assignment.

What you can do here is:

static void regCommand(String command, Function action, {bool? isMustRemove}) {
      teleDart.onCommand(command).listen((message) {
          if (isMustRemove ??= false) {
              teleDart.deleteMessage(message.chat.id, message.message_id);
          }
    });
  }

instead of splitting the assignment and the reading.

[asashour]

This was requested in dart-lang/language#1536

[lrhn]

Good find. Closing this as duplicate.