Pattern matching invalid never matches

[Zekfad]

Following code produces composite and doesn't report dead code. Annotations file is too large to post and it's hard to dissect exact class hierarchy, so I've attached it as a text file.

void main(List<String> args) {
  pack.NumberType type = pack.CompositeIntegerType.int16;

  switch(type) {
    case pack.CoreNumberType():
      print('core');
    case pack.CompositeIntegerType():  // The matched value type 'NumberType' can never match the required type 'CompositeIntegerType'.
      print('composite');
  }
}

annotations.dart.txt

[lrhn]

What do you expect the code to do?

The subtypes of NumberType are declared, if I read it correctly, as:

sealed class NumberType implements Comparable<NumberType>  {...}
sealed class CoreNumberType implements NumberType {}
sealed class IntegerType implements NumberType {...}
sealed class FloatType implements NumberType {}
enum CoreIntegerType with _NumberTypeCompare implements IntegerType, CoreNumberType {...}
enum CompositeIntegerType with _NumberTypeCompare implements IntegerType {...}
enum CoreFloatType with _NumberTypeCompare implements FloatType, CoreNumberType {}

That is quite a sealed hierarchy, so we shouldn't need to worry about any subclasses from other libraries.

The enum value, CompositeIntegerType.int16's runtime type is CompositeIntegerType, so it should match the case CompositeIntegerType():.

The case CoreNumberType() exhaust both CoreIntegerType and FloatType (because the latter is sealed, and CoreNumberType exhausts the only subtype CoreFloatType).

[Zekfad]

Code works as expected, but analyzer reports that pattern is unmatchable which is not true:

[lrhn]

Thanks. Definitely does not look right.

Pasting this main function into the annotations.dart file, and putting it into DartPad, does give the warning:

warning line 261 • The matched value type 'NumberType' can never match the required type 'CompositeIntegerType'. Try using a different pattern.

and running it prints "composite", so the analyzer is wrong.

[denniskaselow]

As a bit of extra weirdness, the following code shows the same The matched value type 'Foobar' can never match the required type 'Foo'. warning, but uncommenting the Thing-class and case removes the warning. Seems like this warning only occurs if it's only enums implementing the sealed class.

void main() {
  Foobar foobar = Foo.foo;
  final result = switch (foobar) {    
    Foo foo => 'foo: $foo',
    Bar bar => 'bar: $bar',
//     Thing thing => 'this is thing $thing',
  };
  print(result);
}

sealed class Foobar {}
enum Foo implements Foobar { foo; }
enum Bar implements Foobar { bar; }
// class Thing implements Foobar {}

[Velkamhell97]

So it is a bug of the analyzer ?

[btrautmann]

I have what I think is a similar (the same?) case that does not involve enums but extension types:

extension type TaxId(String value) {
  bool get isValid {
    return value.length == 9;
  }
}

extension type DateOfBirth(DateTime value) {
  bool get isValid {
    return value.isBefore(DateTime.now());
  }

  String get formatted {
    final format = DateFormat(DateTimeFormat.userInput);
    return format.format(value);
  }
}

class HumanRelationship extends BeneficiaryRelationship {
  final DateOfBirth dateOfBirth;

  HumanRelationship({required this.dateOfBirth});
}

class NonHumanTrustRelationship extends BeneficiaryRelationship {
  final TaxId taxId;

  NonHumanTrustRelationship({required this.taxId});
}

class NonHumanEntityRelationship extends BeneficiaryRelationship {
  final TaxId taxId;

  NonHumanEntityRelationship({required this.taxId});
}

Both warnings are:

The matched value type 'TaxId' can never match the required type 'TaxId'.
Try using a different pattern.dart(pattern_never_matches_value_type)

legal_account_beneficiary_draft.dart(78, 16): TaxId is defined in <Location A>
legal_account_beneficiary_draft.dart(78, 16): TaxId is defined in <Location A>

I find it interesting that the warning indicates 2 declaration locations of the extension type but both are the same.

Dart SDK version: 3.3.1 (stable) (Wed Mar 6 13:09:19 2024 +0000) on "macos_arm64"

[eernstg]

I believe the original unjustified warning has been fixed in the bleeding edge version, and this issue can be closed. At least, I don't see that warning when running a fresh analyzer on the original example. @scheglov, do you agree?

For the new comment, @btrautmann, I think it is the same issue and it would also be fixed today.

Example program, doesn't produce any warnings, runs

```dart import 'package:intl/intl.dart'; // import 'package:intl4x/darttime_format.dart'; // Glue code. class DateTimeFormat { static String get userInput => ''; } sealed class BeneficiaryRelationship {} class Beneficiary { final BeneficiaryRelationship relationship; Beneficiary(this.relationship); } // End glue code. extension type TaxId(String value) { bool get isValid { return value.length == 9; } } extension type DateOfBirth(DateTime value) { bool get isValid { return value.isBefore(DateTime.now()); } String get formatted { final format = DateFormat(DateTimeFormat.userInput); return format.format(value); } } class HumanRelationship extends BeneficiaryRelationship { final DateOfBirth dateOfBirth; HumanRelationship({required this.dateOfBirth}); } class NonHumanTrustRelationship extends BeneficiaryRelationship { final TaxId taxId; NonHumanTrustRelationship({required this.taxId}); } class NonHumanEntityRelationship extends BeneficiaryRelationship { final TaxId taxId; NonHumanEntityRelationship({required this.taxId}); } void main() { var beneficiary = Beneficiary(NonHumanTrustRelationship(taxId: TaxId("123456789"))); final subtitle = switch (beneficiary.relationship) { HumanRelationship(:final dateOfBirth) => dateOfBirth.formatted, NonHumanTrustRelationship(:final taxId) => taxId.value, NonHumanEntityRelationship(:final taxId) => taxId.value, }; print(subtitle); } ```

[scheglov]

Support for extension types was fixed as https://github.com/dart-lang/sdk/issues/54561.

[eernstg]

Very good, I'll close the issue.