New lint to remove ?? inside if null test when the body does more things

[FMorschel]

Repro:

class A {
  int? value;
  void foo() {
    if (value == null) {
      value ??= 0;
    }
  }
}

This is a code that triggers prefer_conditional_assignment with a quick-fix that suggests removing the if.

class A {
  int? value;
  void foo() {
    if (value == null) {
      value ??= 0;
      print(value);
    }
  }
}

This code is the same but it does more things, so in this case I believe we should have a new lint + quick-fix for reminding the user to remove the ?? from the assignment expression since it doesn't do anything.

As the bot reminded me to be explicit: the second code has no lint today.

[dart-github-bot]

Summary: The prefer_conditional_assignment lint incorrectly suggests removing an if statement when the body contains additional code besides the conditional assignment. A new lint should be created to detect and suggest removing the ?? operator in such cases.

[srawlins]

Yeah not a bad idea. I think we need information about whether a promotable variable is definitely null (or Null type). But I think currently we don't promote on == null. I think there is a language issue for this proposal.

[FMorschel]

Do you mean dart-lang/language#3031 or dart-lang/language#3057?

[srawlins]

This one: dart-lang/language#1505