Code fixer adds async keyword but does not change return type from object to Task<object>

[thomhurst]

Description

The NUnit migration code fixer (TUNU0001) adds the async keyword to methods but fails to update the return type when the original return type is object. This results in a compilation error because async methods cannot return object directly - they must return Task, Task<T>, ValueTask, or ValueTask<T>.

Reproduction Steps

1. Create a file with a method returning object that contains assertions:

using NUnit.Framework;

public abstract class BaseClass
{
    public abstract object Invoke();
}

public class TestClass : BaseClass
{
    public override object Invoke()
    {
        Assert.NotNull("test");
        Assert.AreEqual(1, 1);
        return "value";
    }
}

2. Run the code fixer: dotnet format analyzers --severity info --diagnostics TUNU0001

3. The code is converted to:

using System.Threading.Tasks;

public abstract class BaseClass
{
    public abstract object Invoke();
}

public class TestClass : BaseClass
{
    public override async object Invoke()  // ERROR: async methods cannot return object
    {
        Assert.NotNull("test");
        await Assert.That(1).IsEqualTo(1);
        return "value";
    }
}

4. Building produces error:

error CS1983: The return type of an async method must be void, Task, Task<T>, ValueTask, or ValueTask<T>

Expected Behavior

When the code fixer adds the async keyword to a method, it should also update the return type:

• object → Task<object>
• int → Task<int>
• etc.

The correct conversion should be:

public override async Task<object> Invoke()
{
    Assert.NotNull("test");
    await Assert.That(1).IsEqualTo(1);
    return "value";
}

Note: The base class declaration would also need to be updated, or the code fixer should recognize that changing the return type would break inheritance and handle this case specially.

Environment

• TUnit Version: 1.11.64
• .NET SDK: 10.0.100-preview.1.25103.12
• OS: Windows 11

Additional Context

This issue was found in a real codebase where a class overrides a method from an abstract base class. The method contains NUnit assertions that were converted to TUnit's async assertions, but the return type was not updated.

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Note: Please add tests when fixing this issue to prevent regression.