Generic test method with a null and a non-null value for one type parameter infers object in source-gen but int in reflection (null becomes 0) #7155

Summary

Confirmed on main (04786b3). A generic test method whose type parameter gets one non-null value and one null infers a different type argument in each engine mode. Source-gen mode runs Run<object>(1, null). Reflection mode runs Run<int>(1, null) and silently turns the null into 0.

Repro

public class GenericNullRow
{
    [Test]
    [Arguments(1, null)]
    public void Run<T>(T a, T b)
    {
        Console.WriteLine($"T={typeof(T).Name} a={a} b={b ?? (object)"<null>"}");
    }
}

Run with --treenode-filter "/*/*/GenericNullRow/*", with and without --reflection.

Observed

Both pass, but the two modes produce different test names (and so different test UIDs and filters). In reflection mode the test body also receives a value that the data row never supplied: null becomes default(int).

Expected

Both modes agree. Reflection mode should not change null into 0 without saying so. Either:

• infer object (what source-gen does, and consistent with null-only rows inferring object), or
• fail the row with a clear message that T was inferred as a value type and cannot accept null. Note that C# itself rejects Run(1, null) for this signature, because it infers T = int and null does not convert to int.

Where

• src/TUnit.Engine/Services/TestGenericTypeResolver.cs:450-464: ResolveMethodGenericArguments skips null arguments, so the non-null 1 alone maps T to int.
• The reflection invoker then passes null into an int parameter, which reflection invocation turns into 0.
• The source generator infers object for the same row.

Related

• Source generator crashes with TUNIT0999 (NullReferenceException) for [Arguments(null)] on a generic test method #7120 / PR fix(engine): infer object for null rows on generic test methods in reflection mode #7142: PR fix(engine): infer object for null rows on generic test methods in reflection mode #7142 changes how reflection mode handles nulls for generic method parameters, so that a type parameter that received only nulls maps to object. Its description says "a non-null value for the same type parameter still wins", so it does not appear to change this mixed case. This repro was run on current main, without fix(engine): infer object for null rows on generic test methods in reflection mode #7142.
• Untyped [Arguments(null)] method row on a generic class silently produces no test #7141: a related null-row shape on generic classes.