[Bug]: [AssertionExtension] on generic class with concrete receiver emits two angle-bracket lists

[JohnVerheij]

Description

When an [AssertionExtension]-decorated class introduces its own generic type parameter and derives from Assertion<TConcrete> for a non-sealed concrete receiver, AssertionExtensionGenerator emits the receiver's covariant type parameter and the class's own generic parameter as two adjacent angle-bracket lists in the generated extension method signature.

Expected Behavior

The generator should emit a single merged generic-parameter list (for example <TActual, T>) so the generated extension compiles.

Actual Behavior

The generator emits something shaped like <TActual><T>, with two separate generic-parameter blocks back-to-back. This is invalid C# syntax; the file fails to parse.

Steps to Reproduce

1. Declare an [AssertionExtension] class that introduces a generic parameter and derives from Assertion<NonSealedConcreteType>:

[AssertionExtension("Matches")]
public class MatchesAssertion<T> : Assertion<System.Exception>
{
    private readonly System.Func<T, bool> _predicate;

    public MatchesAssertion(AssertionContext<System.Exception> context, System.Func<T, bool> predicate)
        : base(context) { _predicate = predicate; }

    protected override System.Threading.Tasks.Task<AssertionResult> CheckAsync(
        EvaluationMetadata<System.Exception> metadata)
        => System.Threading.Tasks.Task.FromResult(AssertionResult.Passed);

    protected override string GetExpectation() => "to match";
}

2. Build the project. The generated extension method has two adjacent <...> blocks in its signature and the build fails.

TUnit Version

1.44.0

.NET Version

.NET 10.0

Operating System

Windows

IDE / Test Runner

dotnet CLI (dotnet test / dotnet run)

Error Output / Stack Trace

error CS1003: Syntax error, ',' expected

Additional Context

The bug surfaces when both conditions hold:

1. The assertion class declares its own generic type parameter (class MyAssertion<T> : ...).
2. The base assertion type is Assertion<TConcrete> where TConcrete is a non-sealed class (i.e. a covariance candidate per the generator's existing covariance logic).

A class that derives from Assertion<TConcrete> for a sealed type, or a class without its own generic parameter, is not affected.

IDE-Specific Issue?

• I've confirmed this issue occurs when running via dotnet test or dotnet run, not just in my IDE