A custom ArgumentDisplayFormatter is being ignored. Here's a minimal reproduction with a failing test because of it (here's a repo with the repro):

public class Tests
{
    [Test]
    [MethodDataSource(nameof(Data1))]
    [ArgumentDisplayFormatter<FooFormatter>]
    public void Test1(Foo foo)
    {
    }

    public IEnumerable<Foo> Data1() => [new()];
}

public class Foo
{
    public override string ToString() => throw new Exception("Foo.ToString");
}

public class FooFormatter : ArgumentDisplayFormatter
{
    public override bool CanHandle(object? value) => true;

    public override string FormatValue(object? value) => "FooFormatterValue";
}

The test fails because the FooFormatter isn't being used and the Foo.ToString() method gets called and throws:

PS D:\Projects\others\TUnitBugs> dotnet test
Restore complete (0.4s)
  TUnitBugs succeeded (0.2s) → TUnitBugs\bin\Debug\net9.0\TUnitBugs.dll
  TUnitBugs test failed with 2 error(s) (0.4s)
    D:\\Projects\\others\\TUnitBugs\\TUnitBugs\\Tests.cs(3): error test failed: 
      Test1 (0ms): InvalidOperationException: Failed to expand data source for test 'Test1': Foo.ToString
      
    D:\Projects\others\TUnitBugs\TUnitBugs\bin\Debug\net9.0\TUnitBugs.dll : error run failed: Tests failed: 'D:\Projects\others\TUnitBugs\TUnitBugs\bin\Debug\net9.0\TestResults\TUnitBugs_net9.0_x64.log' [net9.0|x64]

Test summary: total: 1, failed: 1, succeeded: 0, skipped: 0, duration: 0.3s
Build failed with 2 error(s) in 1.1s

I found that this happens because the argument display value is computed before the formatter is added.

The argument display value is being created here:

Which happens before the events are executed for the test later:

And ArgumentDisplayFormatterAttribute depends on having it's OnTestDiscovered executed so it could add the argument formatter.

I discovered this, because I have a class that throws for the default argument formatter, so I'm using a custom one to make sure my tests don't fail. Except right now the tests are failing anyways.