Mixin coverage incorrectly reported when mixin is only applied in tests

[jellynoone]

General description:
When a mixin is provided by a library but only applied in tests, coverage isn't reported for these mixins.

Meaning, a mixin only has its coverage reported if it is applied in the package and then the class applying it used in tests.

Consider the following example files:

# pubspec.yaml
name: _coverage_test
publish_to: none

environment:
  sdk: '>=2.19.4 <3.0.0'

dev_dependencies:
  test: ^1.23.1

// lib/lib.dart
library _coverage_test;

mixin CalculatorMixin {
  int calculate() => 10;
}

abstract class CalculatorBase {
  int calculate() => 10;
}

class AppliedCalculatorMixin with CalculatorMixin {}

class AppliedCalculatorBase with CalculatorBase {}

and

// test/lib_test.dart
import 'package:_coverage_test/lib.dart';
import 'package:test/test.dart';

void main() {
  group('$CalculatorMixin', () {
    test('calculate()', () {
      expect(TestAppliedCalculatorMixin().calculate(), 10);
      // expect(AppliedCalculatorMixin().calculate(), 10);
    });
  });
  group('$CalculatorBase', () {
    test('calculate()', () {
      expect(TestAppliedCalculatorBase().calculate(), 10);
      // expect(AppliedCalculatorBase().calculate(), 10);
    });
  });
}

class TestAppliedCalculatorMixin with CalculatorMixin {}

class TestAppliedCalculatorBase with CalculatorBase {}

If we run:

rm coverage/**/*.* ; \
dart pub global run coverage:test_with_coverage -o coverage && \
dart pub global run coverage:format_coverage --lcov --check-ignore --in=coverage --out=coverage/coverage.lcov --report-on=lib && \
genhtml coverage/coverage.lcov -o coverage/html

coverage is reported as: lines......: 0.0% (0 of 2 lines)

If we, however, uncomment:

• expect(AppliedCalculatorMixin().calculate(), 10); and,
• expect(AppliedCalculatorBase().calculate(), 10);
  and re-run the previous command, coverage is reported as being 100% (as expected).

Expected behaviour
Coverage should be correctly reported for mixins even when they are only applied in test files.

Additional note
Implementing TestAppliedCalculatorBase as class TestAppliedCalculatorBase extends CalculatorBase {} reports correct coverage for CalculatorBase even when only used in tests.

dart --version: Dart SDK version: 2.19.4 (stable) (Tue Mar 7 09:58:46 2023 +0000) on "macos_x64"

[jellynoone]

Further tested this with:
Dart SDK version: 3.0.0-290.3.beta (beta) (Tue Mar 21 16:51:50 2023 +0000) on "macos_x64" and it is working as expected i.e. the line coverage is correctly reported.

On the other hand, I tested this with:
Dart SDK version: 2.19.5 (stable) (Mon Mar 20 17:09:37 2023 +0000) on "macos_x64" and the line coverage is not correctly reported.

Additionally, it seems that this issue is the same as one reported at #49887.

cc @liamappelbe as you seemed to have resolved the previous one.

[liamappelbe]

That's odd. Seems like it could be the same bug as #49887, but the fix for that landed in 2.18.3. It's good that this new bug seems to be fixed in 3.0.0, but I'd like to understand why it's happening in 2.19.5.