local variable looses promoted type in closure, in the presence of a re-assignment

[srawlins]

(Note I have a very amateur understanding of flow analysis)

Take this code:

void f(String? s, List<int> lengths) {
  for (;;) {
    if (s == null) return;
    
    if (1==2) break;
    s = "not null";
  }
  s.length; // OK
  
  // The property 'length' can't be unconditionally accessed because the receiver can be null.
  lengths.any((e) => e == s.length);
}

So after the for loop, s is promoted to String. s.length is then OK. But later on, in a closure, s.length is not OK; analyzer and CFE say that s is a String?.

I know that there is not a lot of flow analysis or promotion going on within closures, but there is some. In this simpler example, which contains zero re-assignments, we get no errors:

void f2(String? s, List<int> lengths) {
  if (s == null) return;
  
  lengths.any((e) => e == s.length);
}

So I'm guess something about the re-assignment (with a String value, note) throws a lot of flow analysis or promotion out the window.

Bug? Feature? I perused for other promotion bugs but didn't find anything like this.

[srawlins]

cc @jeffkwoh @stereotype441

[srawlins]

(I'm looking at this in DartPad, by the way, using Dart 2.13.1)

[jeffkwoh]

Thank you for reproducing and filing this here :)

[leafpetersen]

Duplicate of #1536 .

[srawlins]

Thanks for finding that, Leaf!