Incorrect unreachable case warning with nullable matched value type and list pattern

[munificent]

If I analyze:

main() {
  List<int>? maybeList;
  switch (maybeList) {
    case [var element]:
      print(element + 1);
    case var otherwise?:        // line 6
      print(otherwise.length);  // line 7
  }
}

I get:

warning • temp.dart:7:7 • Dead code. Try removing the code, or fixing the code before it so that it can be reached.
          • dead_code
   info • temp.dart:6:5 • This case is covered by the previous cases. Try removing the case clause, or restructuring
          the preceding patterns. • unreachable_switch_case

It apparently thinks that the preceding case's list pattern covers... everything? This is doubly wrong: The first case won't match null, nor will it match non-null lists of any length other than one.

It seems to have something to do with the matched value type being nullable. If I change it to:

main() {
  List<int> maybeList = [];
  switch (maybeList) {
    case [var element]:
      print(element + 1);
    case var otherwise:
      print(otherwise.length);
  }
}

Then there are no warnings.

[johnniwinther]

@stereotype441 I think the problem is in the flow analysis. It seems like maybeList is promoted to Never? via the "unmatched" path through [var element].

[stereotype441]

Good catch, @munificent! And yes, @johnniwinther, your analysis is correct. I dug through the code, and the problem occurs whenever the following are all true of a list pattern:

• The matched value type is List<U>? for some U.
• The element type of the list pattern is V, where U <: V (meaning the only way that the type check implied by the list pattern can fail is if the matched value is null).
• The list pattern doesn't match all possible list lengths.

When this happens, flow analysis incorrectly uses the ifFalse control flow branch of the type test as the "unmatched" control flow path for the pattern. In other words, it fails to account for the possibility that the list pattern might fail to match due to the list having an incorrect length. As a result, it promotes the matched value type to Never? (which is equivalent to Null) in the "unmatched" control flow path.

I'll work on a fix ASAP.

[stereotype441]

Fix is out for review: https://dart-review.googlesource.com/c/sdk/+/364942