Coverage fails when extension block includes method with arg default

[superstator]

Describe the bug
When my project includes an extension block which contains a method with an argument with a default value, the project builds and tests pass, but the output coverage.cobertura.xml file is empty.

To Reproduce

With this code in the target project:

extension(LogLevel level)
{
    // real implementation does more, obviously, but this is sufficient to reproduce the problem
    public static LogLevel Parse(string? logLevel, LogLevel defaultLogLevel = LogLevel.Information) =>
       LogLevel.Information;
}

coverage.cobertura.xml looks like this:

<?xml version="1.0" encoding="utf-8"?>
<coverage line-rate="0" branch-rate="0" version="1.9" timestamp="1787695450" lines-covered="0" lines-valid="0" branches-covered="0" branches-valid="0">
  <sources />
  <packages />
</coverage>

Whereas if I remove the default value for defaultLogLevel:

extension(LogLevel level)
{
    // real implementation does more, obviously, but this is sufficient to reproduce the problem
    public static LogLevel Parse(string? logLevel, LogLevel defaultLogLevel) =>
       LogLevel.Information;
}

I get detailed information in coverage.cobertura.xml

<?xml version="1.0" encoding="utf-8"?>
<coverage line-rate="1" branch-rate="0.8887999999999999" version="1.9" timestamp="1787695430" lines-covered="51" lines-valid="51" branches-covered="16" branches-valid="18">
...snip

Expected behavior

Full coverage results are produced even if methods in extension blocks contain default values for arguments.

Actual behavior

Coverage results are empty when extension blocks contain default values for arguments.

Configuration:

Test project csproj:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <IsPackable>false</IsPackable>
    <AnalysisMode>Recommended</AnalysisMode>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="coverlet.collector" Version="10.0.1">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
    </PackageReference>
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.9.0" />
    <PackageReference Include="NUnit" Version="4.6.1"/>
    <PackageReference Include="NUnit.Analyzers" Version="4.14.0">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
    </PackageReference>
    <PackageReference Include="NUnit3TestAdapter" Version="6.3.0" />
  </ItemGroup>

  <ItemGroup>
    <Using Include="NUnit.Framework"/>
  </ItemGroup>

  <ItemGroup>
    <ProjectReference Include="<cool target project>.csproj"/>
  </ItemGroup>

</Project>

Additional context

I am on Ubuntu 24.04, with .NET SDK 10.0.302. I have not tried to reproduce on Windows or MacOS, but we do see the same failure to generate coverage reports on GitHub Actions runners.

[Bertk]

Thank you for the information.

Please ensure you are using the .NET 10 vstest configuration; see Testing with dotnet test.

As a quick check, try running the tests with .NET 9. If that works, the issue may be caused by a missing .NET 10 vstest configuration.

For .NET 10, we recommend using the coverlet.MTP package.