Wrong flow analysis after non-nulable type and `==null`

[sgrekhov]

The following code works well and produces no errors in both CFE and the analyzer.

main() {
  late int i;
  String s = "";
  if (s == null) {  // Warning: unnecessary_null_comparison
    i = 42;
  }
  i; // No error
}

But according to the flow analysis specification:

operator== If N is an expression of the form E1 == E2, where the static type of E1 is T1 and the static type of E2 is T2, then:
...

• Otherwise, if equivalentToNull(T1) and T2 is non-nullable, or equivalentToNull(T2) and T1 is non-nullable, then:
  • Let true(N) = unreachable(after(E2)).
  • Let false(N) = after(E2).

According to the above, the body of if (s == null) ... is unreachable and therefore initialization of i occurs in a dead code. This leaves i definitely unassigned and therefore i; should be a compile-time error.

Probably this happens because s == null is not always false in an unsound mode, but there are no mentions of unsound mode in the flow analysis spec.

cc @johnniwinther

Dart SDK version: 3.8.0-73.0.dev (dev) (Thu Feb 6 20:02:31 2025 -0800) on "windows_x64"

[dart-github-bot]

Summary: Flow analysis incorrectly handles late variable assignment within an if (s == null) block where s is a non-nullable String. The analyzer doesn't flag the subsequent use of the late variable as an error, despite it potentially being unassigned.

[sgrekhov]

Seems that the below is one more demonstration of the same issue.

void test(int x) {
  late int i;
  x ?? (i = 42); // Warning: dead_null_aware_expression
  i; // Definitely unassigned, but no expected error
//^
// [analyzer] unspecified
// [cfe] unspecified
}

[stereotype441]

@sgrekhov

Sorry for the slow response; this got buried in my inbox.

Probably this happens because s == null is not always false in an unsound mode, but there are no mentions of unsound mode in the flow analysis spec.

You are exactly right, that's the reason why the body of if (s == null) ... isn't flagged as unreachable in your example. I'll update the spec to match the implementation, and include a comment explaining the rationale.

Note that now that support for unsound null safety has been completely removed (0060b0f), I intend to change this behavior to match what the spec currently says. But since we're past the Beta 2 cutoff for Dart 3.8, I won't be doing it until Dart 3.9 at the earliest. See dart-lang/language#3100.

[stereotype441]

You are exactly right, that's the reason why the body of if (s == null) ... isn't flagged as unreachable in your example. I'll update the spec to match the implementation, and include a comment explaining the rationale.

Proposed spec change is here: dart-lang/language#4284