Type Promotion For If-Case Statements

[mcmah309]

For sealed types with two implementations, Dart should be able to promote the type based on if-case statements, like it does with switch statements. Example given

sealed class Result<S,F> {}

final class Ok<S,F> implements Result<S,F>{
  final S ok;
  
  const Ok(this.ok);
}

final class Err<S,F> implements Result<S,F>{
  final F err;

  const Err(this.err);
}

Result<int,String> okResult() => Ok(1);

This works

Result<int,String> main(){
  Result<int,String> result = okResult();
  switch(result){
    case Err():
      return result;
    case Ok():
  }
  int s = result.ok; // type promotion works
 return Ok(s+1);
}

But this does not

Result<int,String> main(){
  Result<int,String> result = okResult();
  if(result case Err()){
    return result;
  }
  int s = result.ok; // type promotion does not work here
  return Ok(s + 1);
}

[mcmah309]

@eernstg based on our current chat here dart-lang/sdk#55104 , I assume the reason this works in the first instance is because specifying all cases of the sealed class triggers exhaustiveness analysis. While this is not currently done for if case statements?

[eernstg]

Right, if-case chains do not have an exhaustiveness property, no attempt is made to detect any such thing.

I don't think we're going to have it either. @stereotype441, WDYT?

[lrhn]

Agree, we do not try to handle exhaustiveness with if except for union types, where we know that there are precisely two immediate subtypes.

We could do the same for sealed types with two immediate subclasses, but that's a feature which feels partial if it stops working at three subclasses.

We could aim to solve the general problem: Given a sealed type with subtypes T1...Tn, can we treat it as a union type T1|...|Tn and have type checks elimintate some parts of it.
(But then we run into the case of T4 being another sealed type, and wanting to eliminate that one subclass at a time. Possibly doable, likely not trivial.)