Null safety feedback: Null check required after checking a final field

[vanlooverenkoen]

Why is this giving an error? The string is final so it will never be null.

void main() {
 
  final test = SomeClass(null);
  test.printPassword();
}

class SomeClass{
  final String? password;
  
  SomeClass(this.password);
  
  void printPassword(){
    if (password == null) {
      print('Password is null');
    }else{
      print(password + ' should never be null');
    }
  }
}

[lrhn]

Dart only ever promotes local variables. You can't know for sure that a virtual getter always returns the same value, even if it's a final field. A subclass can override the getter with something else, and that subclass might be declared in a completely different package.
Dart only promotes when it can prove that the value you test is also the same value that you are using later, based only on the signatures. It can only do that for local variables.

[natebosch]

Closing since this is a duplicate and there is more in-depth conversation in other issues.
field-promotion Issues related to addressing the lack of field promotion