Analyzer and CFE disagree on type promotion due to boolean condition variables in pattern assignments

[stereotype441]

Background: Dart has a feature whereby a boolean condition can be stored in a variable and later used to control type promotion, e.g.:

f(int? x) {
  bool b;
  // ...
  b = x != null;
  // ...
  if (b) {
    print(x + 1); // OK; o has been promoted to `int`.
  }
}

When I added flow analysis support for pattern assignments, I tried to make pattern assignments have the same behaviour, e.g.:

f(int? x) {
  bool b;
  // ...
  (b) = x != null; // Note: pattern assignment
  // ...
  if (b) {
    print(x + 1); // OK; o has been promoted to `int`.
  }
}

But it looks like it doesn't work properly. The analyzer accepts this second example; the CFE does not.

Reproduced in c13acbf.

[stereotype441]

I'm investigating now.

[stereotype441]

Found the problem. The CFE generally uses the convention that the expressions passed to flow analysis are lowered expressions (that is, the expressions that remain after the rewrites that occur during type inference), whereas the shared analysis code for patterns follows the convention of passing pre-lowered expressions to flow analysis. Because of this mismatch, flow analysis is unable to keep track of the expression x != null (since it gets lowered by the CFE to !(x == null)), and so the promotion is lost.

I'm preparing a short-term workaround (which I'll upload for code review soon). In the long run I would like to rework the API for flow analysis so that the expressions passed to it are always the pre-lowered expressions. This should make it a lot easier to ensure consistency between the analyzer and the front end, since the analyzer hardly does any lowering at all. I've filed #52189 to track this longer-term work.

[chloestefantsova]

In case it might be helpful, I worked on a similar bug some time ago: https://dart-review.googlesource.com/c/sdk/+/287601