Flow analysis. Reachability in case of null-aware method invocation.

[sgrekhov]

Now we have the following.

class C {
  void foo(int x) {}
}

test1(C? c) {
  late int i;
  if (c == null) {
    c?.foo(i = 42); // No dead code warning
  }
  i; // No error
}

test2(C? c) {
  int i;
  if (c != null) {
    c?.foo(i = 42); // Warning. The receiver can't be null, so the null-aware operator '?.' is unnecessary.
  }
  i; // Error. Not definitely assigned.
}

main() {
  test1(C());
  test2(C());
}

I think it's ok for now because sound-flow-analysis feature is not implemented yet. @stereotype441 what should we expect with this feature? I'd expect dead code warning and "Definitely unassigned" error in test1().

I'm not sure about what expect in case of test2(). How flow analysis should treat if (c != null) { c?.something;}?

Dart SDK version: 3.9.0-43.0.dev (dev) (Tue Apr 22 13:03:05 2025 -0700) on "windows_x64"

[stereotype441]

With sound-flow-analysis enabled, I would expect no change in the behavior of test1. The reason is that flow analysis never promotes to Null. It could do so soundly, but to do so would have negative consequences. For details see dart-lang/language#1505 (comment).

With sound-flow-analysis enabled, I would expect the "not definitely assigned" error in test2 to go away, because if (c != null) promotes c to type C, and ?. applied to a non-nullable type is guaranteed to execute its right hand side.

[sgrekhov]

@stereotype441 thank you for the explanation! The question is answered so I'm closing this issue.