[thomhurst]

Summary

An untyped [Arguments(null)] method row on a generic class no longer disappears. Before this PR, when nothing else supplied the class's type argument, the row produced no test, no diagnostic and no failure, in both modes. It now runs as C<object>, or fails visibly when object breaks the class's constraints. Source-gen and reflection modes behave the same.

public class NullRowOnGenericClass<T>
{
    [Test]
    [Arguments(null)]
    public void Run(T value) { }
}

Fixture (Bugs/_7141)	Before (both modes)	After (both modes)
[Arguments(null)] only	0 tests	1 passed, C<object>
[Arguments(1)] [Arguments(null)]	1 test, null row dropped	2 passed, C<int> and C<object>
where T : struct, [Arguments(1)] [Arguments(null)]	1 test, null row dropped	1 passed, 1 failed: Type 'Object' does not satisfy the 'struct' constraint for generic parameter 'T'
[GenerateGenericTest(typeof(string))] + [Arguments(null)]	1 passed on C<string>	unchanged

Cause

#7106 (PR #7111) made a null params array infer no class type argument, in InferClassTypesFromMethodArguments (generator) and InferClassTypeArgumentsFromMethodRow / ReflectionGenericConstraintViolations (reflection). Such a row runs only on instantiations that something else establishes, such as a [GenerateGenericTest] or a class-level data source. With neither of those, nothing picked the row up, so it was dropped.

Fix

The rule now depends on whether the class type arguments are already supplied. The gate is the same one both modes already use for method-row inference (classTypeArgumentsSupplied in the generator, and in reflection the [GenerateGenericTest] / class-data-source checks):

• Supplied ([GenerateGenericTest] or a class-level data source): unchanged. [Arguments(null)] infers nothing and runs on those instantiations (Generic test classes drop [Arguments] rows that infer the same type arguments (both modes; reflection runs no method rows) #7106 fixtures ClassRowWithUninferredMethodRow, GenerateGenericTestWithUninferredMethodRow and the others still pass).
• Not supplied: the row's one stored null value infers object, the same as a null element ([Arguments(null, 1)]) and the generic-method case (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). It reuses the existing paths: a valid C<object> instantiation runs, and a constraint-breaking one goes through the Generic class type arguments that violate constraints are dropped in source-gen and fail the whole assembly scan in reflection mode #7107 constraint-violation reporting as a failed test.

The generator handles exactly the shape that reflection infers from the single null value: a one-type-parameter class whose first method parameter is that type parameter. It mirrors the generic-method special case in InferTypesFromArgumentsAttribute.

Behaviour change to an existing fixture

_7106/NullArrayMethodRowOnConstrainedClass asserted that the null row on a where T : struct class was dropped silently (1 test). That silent drop is the behaviour this issue reports. The fixture now lives in _7141 as NullRowOnConstrainedGenericClass and expects 1 passed and 1 failed with the constraint message. It has no [EngineTest] attribute, so the [EngineTest=Pass] run does not pick it up.

Related PRs

• fix(engine): infer object for null rows on generic test methods in reflection mode #7142 (Source generator crashes with TUNIT0999 (NullReferenceException) for [Arguments(null)] on a generic test method #7120): the same object inference for generic methods. This PR is independent of it and is based on main.
• fix(sourcegen): report generic test rows whose type arguments cannot be inferred #7187 (Source-gen silently drops a generic test method when its type arguments cannot be inferred from the data source #7163): rows whose generic method types cannot be inferred now fail visibly instead of vanishing. This PR applies the same idea to generic classes: every row becomes either a real test or a failed test.
• fix(generic): infer object when a type parameter gets null and a value-type value #7198 (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): null with a value type infers object for generic methods. Within one row, [Arguments(null)] has a single value, so there is no null/value mix and the two rules do not overlap. InferClassTypeArgumentsFromMethodRow's handling of mixed rows such as [Arguments(1, null)] is unchanged.
• fix(engine): run method rows only on the generic class instantiation they infer #7149 (Generic classes with differently typed [Arguments] rows produce different test sets in source-gen and reflection modes #7118) touches ReflectionTestDataCollector for classes that have class rows. This PR changes only the path without class data sources.

Still not inferred, in both modes, and unchanged: [Arguments(null)] where the first parameter only contains the type parameter (List<T>, or T? with where T : struct), and classes with several type parameters. A null cannot infer those, in a null element row either.

Tests

• New fixtures: tests/TUnit.TestProject/Bugs/_7141/NullArgumentsRowOnGenericClassTests.cs. On main, both modes ran 3 tests. With the fix, both run 6: 5 passed, 1 failed (the intended constraint failure).
• New tests/TUnit.Engine.Tests/Issue7141Tests.cs: 4/4 in reflection mode. AOT mode is skipped locally because it runs only in CI.
• New generator snapshot test Tests7141 (it also asserts that NullRowOnGenericClass<object> is emitted). Verified on net10.0, net9.0, net8.0 and net472.
• Full TUnit.Core.SourceGenerator.Tests on net10.0: 192 passed, 1 skipped, no existing snapshot changed.
• Regression, TUnit.Engine.Tests on net10.0: Issue7106Tests (60), Issue7107Tests, Issue7092Tests, Issue7096Tests, Issue7121Tests, GenericConstraintValidationTests and GenericMethodWithDataSourceTests all pass.

Fixes #7141

Summary by CodeRabbit

• Bug Fixes

  • Improved handling of [Arguments(null)] for generic test classes by inferring object when appropriate.
  • Null argument rows now behave consistently across source-generated and reflection-based test execution.
  • Generic constraint violations are reported correctly when object does not satisfy the constraint.
  • Explicitly generated generic instantiations continue to control which null rows run.
  • Unsupported by-reference generic scenarios are no longer registered incorrectly.

• Tests

  • Added regression coverage for null-only, mixed-value, constrained, and explicitly generated generic test cases.

[coderabbitai]

📝 Walkthrough

[Arguments(null)] now infers object for eligible generic-class test rows in source-generation and reflection modes. The change also preserves explicit generic instantiations, handles constraint violations, and adds regression coverage for mixed rows and by-reference parameters.

Changes: Generic class null argument inference

Layer / File(s)	Summary
Inference behavior across discovery modes src/TUnit.Core.SourceGenerator/Generators/TestMetadataGenerator.cs, src/TUnit.Engine/Discovery/ReflectionTestDataCollector.cs, src/TUnit.Engine/Discovery/ReflectionGenericConstraintViolations.cs, src/TUnit.Core/Attributes/TestData/ArgumentsAttribute.cs	Eligible null rows infer object for generic classes. Inference is disabled when generic type arguments are already supplied or when the parameter is by-reference. Reflection processes null rows for constraint checks. The attribute documentation describes the inference behavior.
Generic class null-row regression coverage tests/TUnit.TestProject/Bugs/_7141/*, tests/TUnit.Core.SourceGenerator.Tests/Bugs/7141/*, tests/TUnit.Core.SourceGenerator.Tests/Tests7141.Test.verified.txt, tests/TUnit.Engine.Tests/Issue7141Tests.cs, tests/TUnit.TestProject/Bugs/_7106/NullMethodRowInferenceTests.cs, tests/TUnit.Engine.Tests/Issue7106Tests.cs	Tests cover null and mixed rows, explicit generic instantiations, by-reference parameters, and struct constraint violations. The prior constrained-class null-row scenario is removed from the #7106 test coverage.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: 🟡 Moderate · up to 22701

A valid generic-class test can still be silently omitted in both discovery modes. Fix this case before merging unless the limitation is explicitly accepted.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 29.41% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 17 functions across 8 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: running untyped [Arguments(null)] rows on generic classes instead of dropping them.
Linked Issues check		[ #7141 ] is directly linked and requires visible handling of untyped [Arguments(null)] rows on generic classes in source-generation and reflection modes. The source generator now infers object fo…
Out of Scope Changes check		The changes stay within [#7141]. Production changes affect generic-class inference, constraint reporting, and related documentation. Test changes add regression coverage and move the obsolete silent-d…

Full details: Linked Issues check

Explanation

[ #7141 ] is directly linked and requires visible handling of untyped [Arguments(null)] rows on generic classes in source-generation and reflection modes. The source generator now infers object for the supported one-type-parameter, first-parameter, non-by-reference case when no class type arguments are supplied. Reflection inference applies the same rule and preserves the supplied-instantiation path. The new _7141 fixtures and generator snapshot cover valid inference, constraint failure, supplied GenerateGenericTest, mixed rows, and by-reference exclusion.

Full details: Out of Scope Changes check

Explanation

The changes stay within [#7141]. Production changes affect generic-class inference, constraint reporting, and related documentation. Test changes add regression coverage and move the obsolete silent-drop fixture to the new expected behavior. The removed _7106 case and updated snapshot support the same behavior change. No unrelated implementation or test changes are demonstrated.

Full details: Docstring Coverage

Explanation

Docstring coverage is 29.41% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 17 functions across 8 files. (1 skipped: 1 unsupported.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

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_{A rabbit found a null row to explore,
And typed its path as object once more.
Mixed rows hopped through, both wide and small,
While struct constraints checked them all.
By-ref rows stayed unregistered,
Then the rabbit thumped: “The tests are ready!”}

_{Comment @coderabbitai help to get the list of available commands.}

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-20T06:01:05.036005Z	4553d25	PR opened

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[chatgpt-codex-connector]

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Reviewed commit: 4553d2595e

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[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge; the previously reported generated-call failure is fixed.

Summary

The PR makes unsupported null-row inference visible on generic classes instead of silently dropping ordinary value-parameter rows.

• Generic class tests run untyped null rows as object when no class type is supplied.

_{Reviews (3) · Last reviewed commit: "fix(sourcegen): leave [Arguments(null)] ..." · Reviewed by Greptile}

[coderabbitai]

Actionable comments posted: 1

---

• 🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at
@src/TUnit.Core.SourceGenerator/Generators/TestMetadataGenerator.cs:
- Line 1126: Update the class type-argument inference around
nullArgumentsArrayInfersObject so a class source without inferred type arguments
is treated as uninstantiated and infers object for null arguments. Ensure both
source-generator discovery and reflection discovery use whether type arguments
were actually inferred, not merely whether a class data attribute exists,
preserving the resulting C<object> test row.

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📥 Commits

Reviewing files that changed from the base of the PR and between c4b9977 and 2270157.

📒 Files selected for processing: 10

• src/TUnit.Core.SourceGenerator/Generators/TestMetadataGenerator.cs
• src/TUnit.Core/Attributes/TestData/ArgumentsAttribute.cs
• src/TUnit.Engine/Discovery/ReflectionGenericConstraintViolations.cs
• src/TUnit.Engine/Discovery/ReflectionTestDataCollector.cs
• tests/TUnit.Core.SourceGenerator.Tests/Bugs/7141/Tests7141.cs
• tests/TUnit.Core.SourceGenerator.Tests/Tests7141.Test.verified.txt
• tests/TUnit.Engine.Tests/Issue7106Tests.cs
• tests/TUnit.Engine.Tests/Issue7141Tests.cs
• tests/TUnit.TestProject/Bugs/_7106/NullMethodRowInferenceTests.cs
• tests/TUnit.TestProject/Bugs/_7141/NullArgumentsRowOnGenericClassTests.cs

💤 Files with no reviewable changes: 1

• tests/TUnit.Engine.Tests/Issue7106Tests.cs

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[github-actions]

Review of #7203

I found no blocking issues. The fix is small and targeted.

• Gate matches in both modes. The generator uses !classTypeArgumentsSupplied. Reflection calls BuildTestsFromMethodRows only when classDataSources.Length == 0 (ReflectionTestDataCollector.cs ~L690) and then checks generateGenericTestAttributes.Length == 0. Together that is the same "nothing else supplies the class type arguments" rule, so the two modes should behave the same.
• By-ref and method-generic guards. The generator restricts the object inference to a single class type parameter used directly as the first parameter type, excluding ref parameters and method-level type parameters. The ByRef assertion in Tests7141 covers this.
• Tests. There are source-generator snapshot tests, engine tests and TestProject fixtures for the cases in the description, including the struct constraint failure.

Suggestions, none blocking:

1. InferClassTypesFromMethodArguments now takes a bool, and the callers pass false in one place and !classTypeArgumentsSupplied in another. A small enum or a named-argument-only overload would make the call sites clearer, and would keep the reflection and generator rules visibly tied to one concept.
2. The rule that a null row infers object now lives in three places: the generator, InferClassTypeArgumentsFromMethodRow and ReflectionGenericConstraintViolations. The latter two depend on the caller's gate staying consistent. A comment cross-referencing them would help prevent drift.

I did not run the test suite. This review is based on reading the diff, and the Skill call for the code-review skill failed.

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