Type promotion failed in named records #55473

yiiim · 2024-04-15T10:35:17Z

Run the following code in DartPad:

void main() {
  int? a;
  Future(
    () {
      ({String b, String c}) test = (
        b: "${a!.isEven}",
        c: "${a.isEven}",
      );
      print(test);
    },
  );
  a = 0;
}

compileDDC
main.dart:7:17: Error: Property 'isEven' cannot be accessed on 'int?' because it is potentially null.
Try accessing using ?. instead.
        c: "${a.isEven}",
                ^^^^^^

lrhn · 2024-04-15T14:08:23Z

This looks like it's working as currently intended. (Edit, narrator voice: It did not.)

Code inside a closure cannot promote a variable if there are assignments to it outside of the closure (maybe only later).
That is the case here.

The fact that the only assignment is to a non-null isn't taken into account. The value may change at a time that is not known to not happen between the a! and the a.isEven, and that's enough to disable promotion.

yiiim · 2024-04-15T14:52:37Z

So is this an error that belongs to the IDE plug-in? I did not get any error prompts in the vscode Dart plug-in v3.86.0.

yiiim · 2024-04-15T14:58:10Z

If it's an unnamed record type, this code can work properly.

void main() {
  int? a;
  Future(
    () {
      (String, String) test = (
        "${a!.isEven}",
        "${a.isEven}",
      );
      print(test);
    },
  );
  a = 0;
}

lrhn · 2024-04-15T22:26:31Z

Good point. It seems like DDC is promoting the variable in one case, so it should in the other too.

johnniwinther · 2024-04-17T08:19:57Z

cc @stereotype441

tp · 2024-04-23T08:08:56Z

In our codebase we had a similar case (and given that it's a closure parameter (which could even be final)) I felt this is a stronger example of the issue:

void main() {
  f((x) => [x!.a, x.b]); // OK
  f((x) => (x!.a, x.b)); // OK
  f((x) => (a: x!.a, b: x.b)); // ERR
}

void f(void Function(X?) c) {
  c(X());
}

class X {
  final a = 1;
  final b = 2;
}

passes dart analyze (which wants one to not add a ! to x.b), but fails compilation with

compileDDC
main.dart:3:33: Error: Property 'b' cannot be accessed on 'X?' because it is potentially null.
 - 'X' is from 'package:dartpad_sample/main.dart' ('/tmp/dartpadTJDUDK/lib/main.dart').
Try accessing using ?. instead.
  f((final x) => (a: x!.a, b: x.b));
                                ^

lrhn · 2024-04-23T11:29:08Z

Seems to be related to information flow between expressions in a record expression that contains named fields.
(As in: it's not there. The promotions of a prior field are not used as starting state for the next field expression, but only if the record expression itself contains named fields.)

void main() {
  ({int x, int y})? a = (x: 42, y: 37) as dynamic;
  var yes = DateTime.now().millisecondsSinceEpoch > 0;
  // Named fields, requires promotion to flow between field expressions.
  // All fail.
  if (yes) {
    print((a!.x, n2: a.y)); // ERR
  }
  if (yes) {
    print((n1: a!.x, a.y)); // ERR
  }
  if (yes) {
    print((n1: a!.x, n2: a.y)); // ERR
  }
  if (yes) {
    print((a!.x, a.y, n3: 0)); // ERR
  }
  // Promotion flow out of record. Works.
  if (yes) {
    print((a!.x, n2: 0));
    print(a.y);
  }
  if (yes) {
    print((n1: a!.x, n2: 0));
    print(a.y);
  }
  // Promotion flow inside field expression. Works.
  if (yes) {
    print((a!.x + a.y, n2: 0));
  }
  if (yes) {
    print((n1: a!.x + a.y, n2: 0));
  }
  // No mamed fields in same record as promotion. Works.
  if (yes) {
    print((a!.x, (n2: a.y)));
    print(a.y);
  }
  if (yes) {
    print((a!.x, a.y));
  }
  // Not records, all work.
  if (yes) {
    print([a!.x, a.y]);
  }
  if (yes) {
    foo(a!.x, a.y);
  }
  if (yes) {
    bar(x: a!.x, y: a.y);
  }
}

void foo([int x = 0, int y = 0]) {
  print("$x,$y");
}

void bar({int x = 0, int y = 0}) {
  print("$x,$y");
}

lrhn added type-bug Incorrect behavior (everything from a crash to more subtle misbehavior) area-fe-analyzer-shared and removed type-bug Incorrect behavior (everything from a crash to more subtle misbehavior) labels Apr 15, 2024

lrhn added the type-bug Incorrect behavior (everything from a crash to more subtle misbehavior) label Apr 15, 2024

johnniwinther added the cfe-dysfunctionalities Issues for the CFE not behaving as intended label Apr 17, 2024

johnniwinther added area-dart-model For issues related to conformance to the language spec in the parser, compilers or the CLI analyzer. model-flow Implementation of flow analysis in analyzer/cfe and removed legacy-area-fe-analyzer-shared labels Apr 1, 2025

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