Call 'String?.length' in a constant expression

[sgrekhov]

The code below produces an error in the analyzer but works in VM. Which tool is right here?

const String? s = null;
const int? c = s?.length; // Analyzer: The property 'length' can't be accessed on the type 'Null' in a constant expression.

main() {
  print(c); // prints null
}

Dart SDK version: 3.7.0-309.0.dev (dev) (Tue Jan 7 20:06:23 2025 -0800) on "windows_x64"

[dart-github-bot]

Summary: Analyzer incorrectly flags String?.length within a constant expression as an error. The VM correctly handles null propagation, resulting in null.

[eernstg]

(Errm, I looked at the summary, but it's wrong. So I deleted my first comment which is not relevant to the actual issue.)

Here's a better comment:

s?.length is never a constant expression, not even if s is a constant expression whose value is null or a String.

So both tools should report an error because s?.length isn't a constant expression (and the error should not be "'length' can't be accessed on the type 'Null'" because that isn't the semantics of ?. in any context).

[lrhn]

Very, very likely e?.id was not made a constant expression in any case because I didn't think it could possibly be valid. There is one case where e.id is constant, constantString.length, and constantStringOrNull?.length could soundly be constant. But it's currently not specified as such.

Maybe we can add it next time we touch the constant sublanguage for some reason, along with is ConstantType. And throw <expr>.