Analyzer: "assert(list.length > 0)" does not produce an error despite it being one

[matanlurey]

This seems like a regression, but I couldn't tell you since when/what version.

$ dart --version
Dart SDK Version: 3.0.5 (stable) (Mon Jun 12 18:31:49 2023 +0000) on "macos_arm64"

void main() {
  const RequiresNonEmptyList([1]);
}

class RequiresNonEmptyList {
  const RequiresNonEmptyList(List<int> numbers) : assert(numbers.length > 0);
}

This cause a constant evaluation error at the call-site const RequiresNonEmptyList([1]), but the actual error is never shown by the analyzer - I have to use another tool (such as Dart2JS) to reveal the root cause:

Error compiling to JavaScript:
Info: Compiling with sound null safety.
lib/main.dart:2:9:
Error: Constant evaluation error:
  const RequiresNonEmptyList([]);
        ^
lib/main.dart:6:66:
Info: The property 'length' can't be accessed on '<int>[]' in a constant expression.
  const RequiresNonEmptyList(List<int> numbers) : assert(numbers.length > 0);

I'm guessing there was plans to allow constant evaluations of <List>.length but it was never fully implemented or requires some sort of flag/experiments that aren't typically enabled - in either case this is confusing behavior to (statically) allow something that is invalid.

For example, if I write : assert(numbers.isNotEmpty), I get (in the analyzer):

Invalid constant value.

(Which isn't a great error message, but at least it's shown within the assert)

[scheglov]

@kallentu

[lrhn]

The plan was to allow String.length as a constant expression, and to allow the parser to recognize compile-time constant expressions without understanding types. That's why e.length is a compile time constant expression if e is one, and a potentially constant expression if e is one, independently of types.

If that potentially constant expression is actually used as a constant (constructor is invoked as const), then e must be a String. Otherwise there is no type requirement.

It's not great, and I'd like to change it at some point, so we reject a potentially constant expression of the form e.length when the static type of e is not assignable to String. But that's a breaking change, and a low priority, so nothing has happened.

The program does have a compile-time error, because [1].length is evaluated as a constant, and the analyzer should report that.

[matanlurey]

Wow, thank you Kallen!