Type promotion on null checks on fields

[gallmerci]

I tried to migrate my flutter code to a null safety version and came across the following problem:

class MyClass {
  final String? myString;
  MyClass(this.myString);
}

String? f(String input) {
  return input;
}

void main(){
  final c = MyClass("string");
  
  String? s = c.myString != null ? f(c.myString) : null;
}

which gives me the following compilation error:

lib/main.dart:13:40:
Error: The argument type 'String?' can't be assigned to the parameter type 'String' because 'String?' is nullable and 'String' isn't.
  String? s = c.myString != null ? f(c.myString) : null; // This 
                                       ^

I can easily change this to an equivalent code by extracting c.myString to a new variable which works fine:

class MyClass {
  final String? myString;
  MyClass(this.myString);
}

String? f(String input) {
  return input;
}

void main(){
  final c = MyClass("string");
  final myString = c.myString;
  String? s = myString != null ? f(myString) : null;
}

It looks like a bug in the type promotion logic in the first example or maybe I am missing something why this couldn't be type promoted :)

[gallmerci]

Duplicate of #1415

[leafpetersen]

Unfortunately, this is working as intended. See here for some general discussion about working with fields, here for some discussion of why even final fields can't be promoted soundly, and here for a list of issues discussing potential ways of making fields easier to work with in null safety.

[gallmerci]

Thanks a lot for the references! Makes sense now 👍