Coverage for compiler-rewritten methods (async, iterator, generic) is dropped, scoring covered code as 0%

[dillon7f]

Summary

Methods the C# compiler rewrites are never matched to their coverage and silently default to 0.0. Because CRAP cubes the uncovered fraction, a fully covered async method can be reported as one of the worst methods in the codebase.

On the project I evaluated, 50 of 60 methods reported over threshold were already covered. The tool's output was ~83% false positives.

Reproduction

PageViewUpdater.UpdateAsync is 100% covered. The tool was handed the coverage file containing that fact and reported:

coverage=0  complexity=18  crap=342

The same file, at the same moment, contains:

<class name="...PageViewUpdater/&lt;UpdateAsync&gt;d__5" line-rate="1">

As a unit test:

[Fact]
public void AsyncMethod_CoverageOnCompilerGeneratedStateMachine_IsAttributedToSourceMethod()
{
    var complexity = new[] { MakeComplexity("UpdateAsync", signature: "(int)") };
    var coverage = new[]
    {
        MakeCoverage("MoveNext", className: "MyApp.Service/<UpdateAsync>d__5", coverage: 1.0)
    };

    var result = MethodCoverageMatcher.Match(complexity, coverage);

    result.Methods.Single().Coverage.Should().Be(1.0);   // fails: 0.0
}

Root cause

CoberturaMethodParser.NormalizeClassName only converts the nested-type separator:

var result = className.Replace('/', '.');

So MyApp.Service/<UpdateAsync>d__5 + method MoveNext produces the key
MyApp.Service.<UpdateAsync>d__5.MoveNext(), which cannot match the source key
MyApp.Service.UpdateAsync(int). Both passes in MethodCoverageMatcher miss and the
left-outer-join default of 0.0 applies.

This affects async methods, iterators, and generic methods. In the project I tested there were 1,014 MoveNext entries in a single coverage file.

Two facets

1. State machines. Recognise Outer/<Method>d__N and attribute MoveNext to the owning method. The generated type carries no parameter list, so the recovered key has no signature and must resolve via the existing name-only fallback pass.

2. Generic arity. The Roslyn side normalizes GetPracticeRights<TWorkflowStatus> to GetPracticeRights<>, while the state machine spells it <GetPracticeRights>d__9`1. The recovered key needs the same backtick-to-angle-bracket conversion MethodKeyHelper.NormalizeBacktickGenerics already performs, or the two still won't meet.

Note on ordering

Fixing this increases the impact of the multiple-coverage-file bug (separate issue), because recovered state-machine keys can only match through the name-only pass, which is exactly the pass that breaks when duplicate entries are present. The two should land together.