8000 Poor problem and correction messages related to assignability · Issue #60516 · dart-lang/sdk · GitHub
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bwilkerson opened this issue Apr 10, 2025 · 1 comment
Open

Poor problem and correction messages related to assignability #60516

bwilkerson opened this issue Apr 10, 2025 · 1 comment
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area-dart-model For issues related to conformance to the language spec in the parser, compilers or the CLI analyzer. model-messages Poor/undesirable messaging in errors/warnings emitted by the analyzer and/or CFE. P3 A lower priority bug or feature request

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@bwilkerson
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Consider the following code:

class A {}

class B implements A {}

class C {
  late A a;

  void m(bool b) {
    a = b ? B() : null; // <--
  }
}

On the marked line there will be a diagnostic of the following form:

A value of type 'B?' can't be assigned to a variable of type 'A'. (invalid_assignment)
Try changing the type of the variable, or casting the right-hand type to 'A'.

While the problem message is technically correct, it would be helpful to have a slightly different message for this particular case, perhaps something of the form

A value of type 'B?' can't be assigned to a variable of type 'A' because it is a nullable type.

The correction, or the second half of it, is not correct. Casting the right-hand side will silence the diagnostic by turning it into a runtime exception. Better advice would be something along the lines of

Try making the variable nullable or change the right-hand to not produce `null`.
@bwilkerson bwilkerson added area-devexp For issues related to the analysis server, IDE support, linter, `dart fix`, and diagnostic messages. P3 A lower priority bug or feature request area-dart-model For issues related to conformance to the language spec in the parser, compilers or the CLI analyzer. and removed area-devexp For issues related to the analysis server, IDE support, linter, `dart fix`, and diagnostic messages. labels Apr 10, 2025
@johnniwinther johnniwinther added the model-messages Poor/undesirable messaging in errors/warnings emitted by the analyzer and/or CFE. label May 8, 2025
@johnniwinther
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cc @chloestefantsova

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area-dart-model For issues related to conformance to the language spec in the parser, compilers or the CLI analyzer. model-messages Poor/undesirable messaging in errors/warnings emitted by the analyzer and/or CFE. P3 A lower priority bug or feature request
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