8000 Call 'String?.length' in a constant expression · Issue #59904 · dart-lang/sdk · GitHub
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sgrekhov opened this issue Jan 14, 2025 · 5 comments
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Call 'String?.length' in a constant expression #59904

sgrekhov opened this issue Jan 14, 2025 · 5 comments
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area-language Dart language related items (some items might be better tracked at github.com/dart-lang/language). type-question A question about expected behavior or functionality

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@sgrekhov
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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"
@sgrekhov sgrekhov added the type-question A question about expected behavior or functionality label Jan 14, 2025
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dart-github-bot commented Jan 14, 2025 •
edited by devoncarew
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Summary: Analyzer incorrectly flags String?.length within a constant expression as an error. The VM correctly handles null propagation, resulting in null.

@dart-github-bot dart-github-bot added legacy-area-analyzer Use area-devexp instead. triage-automation See https://github.com/dart-lang/ecosystem/tree/main/pkgs/sdk_triage_bot. labels Jan 14, 2025
@eernstg
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eernstg commented Jan 14, 2025 •
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(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
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lrhn commented Jan 14, 2025

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>.

@devoncarew devoncarew added area-language Dart language related items (some items might be better tracked at github.com/dart-lang/language). and removed legacy-area-analyzer Use area-devexp instead. triage-automation See https://github.com/dart-lang/ecosystem/tree/main/pkgs/sdk_triage_bot. labels Jan 15, 2025
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lrhn commented Apr 9, 2025 •
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(Question answered. Changing it would be a language feature. Until then s == null ? null : s.length works.)

@lrhn lrhn closed this as completed Apr 9, 2025
@sgrekhov sgrekhov mentioned this issue Apr 9, 2025
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sgrekhov commented Apr 9, 2025 •
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Filed #60509 for the error non reported by CFE

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