Flow analysis. In a do-while loop an assignment in `while` after `break` is dead code

[sgrekhov]

UPD. Seems that according to the #42232 (comment) the below is not an issue but an expected behavior. Please confirm.

The code below produces no compile-time errors in either CFE or the analyzer.

main() {
  late int i;
  do {
    break;
  } while ((i = 42) < 0);
  i; // Definitely unassigned but no compile-time error
}

cc @stereotype441

Dart SDK version: 3.8.0-197.0.dev (dev) (Sat Mar 15 17:07:22 2025 -0700) on "windows_x64"

[stereotype441]

Yes, I can confirm that the fact that there is no compile-time error is expected (though unfortunate) behavior, and agress with the specification. The reason is because when flow analysis reaches the top of the do loop, it needs to use conservativeJoin to do an approximate analysis of what might happen within the loop. Since it hasn't done a full flow analysis of the loop yet, it doesn't see that the loop condition is unreachable, so it marks i as potentially assigned. Later, when it reaches the break; statement, it sees that the loop condition is unreachable, but at that point it is too late to un-do the fact that i has been marked as potentially assigned. So when it reaches i;, it's still considered potentially assigned, and there is no error.

Fixing this for the general case would require flow analysis to make multiple passes over the source code, which we've decided we don't want to do.

Closing as "not planned"