fix: restore null suppression for built-in assertion methods

[thomhurst]

Description

TUnit 1.66.8 reintroduced nullable warnings for collection and async Assert.That(value).IsNotNull() assertions, breaking builds that treat warnings as errors. The validation added in #6700 accepted only AssertionExtensions.IsNotNull, while these assertions resolve to instance methods.

Recognize built-in null checks using the shared collection and async source type symbols and the declaring assembly. This covers specialized collection bases without enumerating their names, while rejecting custom methods that hide inherited members. Require referenced TUnit assertion assemblies so source-defined lookalike types cannot suppress diagnostics.

Validate intermediate assertion members so an external transform returning a TUnit assertion on another value cannot suppress warnings on the original value. Retain warnings for chains containing .Or before or after the null check, since the null check is optional. Both safeguards apply to Assert.That(...).IsNotNull() and Should().NotBeNull().

Related Issue

Regression introduced by #6700; reproduced in thomhurst/Dekaf#3009.

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Checklist

• Read contributing guidelines and followed project code style.
• Added collection regression tests before the fix, in a separate commit.
• Covered arrays, lists, read-only lists, enumerable/collection interfaces, mutable/read-only dictionaries, and sets.
• Covered actual IAsyncEnumerable, Task, and Func overloads, with positive and negative controls.
• Covered CS8600, CS8602, CS8604, properties, foreach, intervening null-forgiving uses, and conjunctions.
• Covered custom/hidden methods, external transforms, disjunctions, and source-defined TUnit lookalikes on both assertion APIs.
• Verified unrelated properties named Or and assertion arguments do not disable valid suppression.

Testing

• Reproduced failures before corresponding fixes: 9 collection cases, 7 chain-safety cases, 3 async cases, and 2 source-lookalike cases.
• Final full TUnit.Assertions.Analyzers.Tests suite: 324 passed, 0 failed (108 each on net8.0, net9.0, net10.0).
• dotnet test --project tests/TUnit.Assertions.Analyzers.Tests --output Normal
• git diff --check passed.

Async tests use the actual framework-specific TUnit libraries with matching framework references: the netstandard2.0 build lacks the async-enumerable entry point. These overloads emit CS8604 at Assert.That(value) and then promote the compiler's null state, so the tests verify that real entry-point diagnostic rather than expecting a later CS8602.

Additional Notes

Analyzer-only change; no public API, source-generator output, engine metadata, or reflection changes. Full repository suite and downstream Dekaf build were not run locally. An existing RS2007 analyzer release-header warning remains.

Summary by CodeRabbit

• Bug Fixes
  • Improved nullability warning suppression after supported IsNotNull() and NotBeNull() assertions, including collection-derived and asynchronous assertion types.
  • Prevented warnings from being incorrectly suppressed when assertion chains include Or, custom transformations, external assertion types, or custom and shadowed methods with similar names.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed2026-09-05T00:02:04.927151Z	6dc75e4	New commits

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[coderabbitai]

📝 Walkthrough

Walkthrough

The analyzer now validates assertion-chain provenance before suppressing nullable warnings. It recognizes supported TUnit collection and asynchronous assertions through their declaring assemblies and shared bases. Tests cover valid assertions and rejected disjunctive, transformed, custom, hidden, and source-defined lookalike methods.

Changes

IsNotNull suppression

Layer / File(s)	Summary
Validate assertion-chain provenance src/TUnit.Assertions.Analyzers/IsNotNullAssertionSuppressor.cs, tests/TUnit.Assertions.Analyzers.Tests/IsNotNullAssertionSuppressorTests.cs	The suppressor rejects optional Or paths and external assertion transformations. Tests cover disjunctive checks, model properties named Or, custom transforms, and updated Or expectations.
Resolve supported IsNotNull methods src/TUnit.Assertions.Analyzers/IsNotNullAssertionSuppressor.cs, tests/TUnit.Assertions.Analyzers.Tests/IsNotNullAssertionSuppressorTests.cs	The suppressor resolves collection-derived and asynchronous assertion methods from referenced TUnit assemblies. Tests cover collection types, asynchronous types, source-defined lookalikes, custom extensions, and hidden methods.

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk:🟡 Moderate · up to 6dc75

This improves nullable-warning suppression for supported assertions, but a custom assertion transform after a null check can still hide a warning for a value that is not ultimately asserted. Validate the complete assertion chain before merge.

Suggested reviewers:mvanhorn

Poem

A rabbit checks the chain,
Or paths do not prove nullness,
TUnit methods stand clear,
Custom shadows lose recognition,
Warnings remain precise.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 19 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the primary change: restoring null suppression for built-in assertion methods.

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[greptile-apps]

Greptile Summary

Restores nullable-warning suppression for built-in collection assertion methods while tightening assertion-chain validation.

• Recognizes collection and async instance IsNotNull methods through their shared TUnit assertion types.
• Rejects source-defined lookalikes, custom transforms, hidden null-check methods, and optional null checks in disjunctive chains.
• Adds regression coverage across collection types, async assertions, both assertion APIs, transforms, and conjunction/disjunction chains.

Confidence Score: 5/5

The PR appears safe to merge; the previous value-tracking, async-coverage, and async-suppression findings are fully addressed.

Chain validation now rejects external transformations before attributing a null check to the original value, and the restored async recognition is covered for async enumerables, tasks, and async delegates. No actionable new defect or repository-rule violation remains.

Important Files Changed

Filename	Overview
src/TUnit.Assertions.Analyzers/IsNotNullAssertionSuppressor.cs	Extends built-in null-check recognition to collection and async assertion types while preventing suppression across external transforms, lookalikes, and disjunctions.
tests/TUnit.Assertions.Analyzers.Tests/IsNotNullAssertionSuppressorTests.cs	Adds broad positive and negative regression coverage for collection, async, transform, assembly-origin, and assertion-chain behavior.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
A[Nullable diagnostic after assertion] --> B{Find Assert.That or Should entry point}
B -- No --> Z[Do not suppress]
B -- Yes --> C{Chain contains Or}
C -- Yes --> Z
C -- No --> D{Intermediate receivers belong to TUnit assertion assemblies}
D -- No --> Z
D -- Yes --> E{Built-in IsNotNull or NotBeNull method}
E -- No --> Z
E -- Yes --> F{Diagnostic expression matches asserted value}
F -- No --> Z
F -- Yes --> G[Suppress nullable diagnostic]

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🧹 Nitpick comments (1)

tests/TUnit.Assertions.Analyzers.Tests/IsNotNullAssertionSuppressorTests.cs (1)

22-32: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Add tests for the async assertion types.

Lines 205-230 add support for AsyncEnumerableAssertionBase and AsyncDelegateAssertion, but this test covers only collection, dictionary, and set assertion types. Add one nullable-warning suppression case for each async Assert.That overload. This will verify both new allowlist branches.

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In `@tests/TUnit.Assertions.Analyzers.Tests/IsNotNullAssertionSuppressorTests.cs`
around lines 22 - 32, Extend
Suppresses_After_Collection_IsNotNull_Instance_Method with nullable-warning
suppression cases for each async Assert.That overload, covering
AsyncEnumerableAssertionBase and AsyncDelegateAssertion. Add one test argument
per async assertion type while preserving the existing collection, dictionary,
and set cases.

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Nitpick comments:
In `@tests/TUnit.Assertions.Analyzers.Tests/IsNotNullAssertionSuppressorTests.cs`:
- Around line 22-32: Extend
Suppresses_After_Collection_IsNotNull_Instance_Method with nullable-warning
suppression cases for each async Assert.That overload, covering
AsyncEnumerableAssertionBase and AsyncDelegateAssertion. Add one test argument
per async assertion type while preserving the existing collection, dictionary,
and set cases.
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[github-actions]

Review: IsNotNullAssertionSuppressor fix for #6700

Traced the full call chain (FindAssertThatInChain, IsTUnitIsNotNullMethod) against every newly added base type and manually simulated it against the new parametrized tests. The core fix is correct: switching from a single hardcoded AssertionExtensions.IsNotNull check to also matching the instance IsNotNull() methods declared directly on the collection/dictionary/set/async base classes correctly restores suppression for Assert.That(list).IsNotNull()-style calls, and the Does_Not_Suppress_After_Custom_IsNotNull_Instance_Method test appropriately guards against user-defined IsNotNull() methods (including ones that hide an inherited member via new) incorrectly suppressing warnings.

Two things worth addressing before/soon after merge:

1. The allow-list is a hardcoded string list, which is exactly how this bug (and presumably #6700) happened in the first place (src/TUnit.Assertions.Analyzers/IsNotNullAssertionSuppressor.cs:220). Every one of the 8 listed types independently redeclares IsNotNull() with the identical body (Context.ExpressionBuilder.Append(".IsNotNull()"); return this;-style pattern), which is itself a sign these should share a single detection mechanism rather than being individually enumerated in the analyzer. The next time a new collection/async source base is added (e.g. FrozenSet, ImmutableList, Queue/Stack), it's easy to forget to add it here and reintroduce CS8602/CS8604 regressions.

Suggested approach: mark the "real" IsNotNull methods with an internal marker, e.g. a [SuppressibleNullCheck] attribute (or an internal marker interface like ITUnitIsNotNullMethod) applied at the point each method is declared in TUnit.Assertions, and have the analyzer check for that attribute/interface instead of a type-name list. This colocates the "this method participates in suppression" decision with the method declaration itself, so adding a new assertion base with its own IsNotNull() automatically works without touching the analyzer — closing off this entire class of regression rather than just patching the current instances.

2. Missing test coverage for 2 of the 8 newly allow-listed types (tests/TUnit.Assertions.Analyzers.Tests/IsNotNullAssertionSuppressorTests.cs:22). The parametrized Suppresses_After_Collection_IsNotNull_Instance_Method test covers 9 collection/dictionary/set argument types, but AsyncEnumerableAssertionBase (IAsyncEnumerable<T>) and AsyncDelegateAssertion (Task/Func<Task>) have no equivalent regression test. If either fully-qualified type string were wrong, or either class were renamed/moved later, CI wouldn't catch it.

Nice to see the regression tests are thorough for the collection cases (including the .And chain, hidden new methods, and property/foreach/assignment dereference sites) — just want the same rigor extended to the async cases so the whole allow-list is actually exercised.

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Actionable comments posted: 1

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Inline comments:
In `@src/TUnit.Assertions.Analyzers/IsNotNullAssertionSuppressor.cs`:
- Around line 223-226: Update the AssertionExtensions check in
IsNotNullAssertionSuppressor to require the referenced real TUnit assembly and
reject symbols whose containing assembly is semanticModel.Compilation.Assembly,
rather than matching only the fully qualified type name. Add a regression test
covering source-defined TUnit.Assertions.Assert and AssertionExtensions
lookalikes so a fake IsNotNull cannot suppress CS8602.
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In `@src/TUnit.Assertions.Analyzers/IsNotNullAssertionSuppressor.cs`:
- Line 330: Update the assertion-chain validation around
IsReferencedAssertionAssembly so it also validates members after IsNotNull(),
rejecting external transforms that replace the original assertion receiver while
still allowing supported terminal task consumption. Preserve suppression only
when the complete chain remains tied to Assert.That(values) or Should(), and add
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[github-actions]

Review

Traced the full suppression logic (IsSupportedAssertionChain, IsTUnitIsNotNullMethod, IsReferencedAssertionAssembly) against the base type hierarchies in src/TUnit.Assertions/Sources and against every new test case. The fix is correct:

• All 9 collection/dictionary/set assertion bases derive from CollectionAssertionBase<,> and hide IsNotNull() with new purely to narrow the return type — the base-type walk in IsTUnitIsNotNullMethod correctly finds CollectionAssertionBase through that hiding, while the assembly-identity check correctly rejects a user's own hidden IsNotNull() on a derived type (verified against the CustomDerivedAssertion test).
• Assert.That(task) / Assert.That(asyncEnumerable) / Assert.That(func) do resolve to AsyncDelegateAssertion / AsyncEnumerableAssertionBase per Assert.cs, matching the new checks, and this path is now covered by Recognizes_Async_IsNotNull_Instance_Methods (added in the latest commit, closing the gap the earlier automated review flagged).
• IsSupportedAssertionChain's two passes are independent and both needed: the "Or" walk starts at the outermost node in the chain and catches Or on either side of the null check, while the assembly-provenance walk (starting at the null check and working back to the entry point) is what actually closes the "custom transform before .IsNotNull()" hole CodeRabbit's risk note called out — confirmed against Does_Not_Suppress_After_Custom_Transform_With_BuiltIn_IsNotNull, where OtherAssertion.IsNotNull() is a legitimately-inherited TUnit method but the transform producing OtherAssertion lives in the user's compilation and correctly fails the check.
• Checked whether Should().NotBeNull() needed the same collection/async instance-method recognition as Assert.That(...).IsNotNull(). It doesnt: per the TUnit.PublicAPI snapshot, NotBeNull is only ever emitted as a single generic IShouldSource<TValue> extension (the generator only wraps extension methods, not hand-written instance methods like the collection bases' hidden IsNotNull()), so the pre-existing IsTUnitMethod check already covers every source type there. Good that this wasnt over-applied.
• The existing .Or chain test correctly flipped from IsSuppressed(true) to IsSuppressed(false) to match the new, more correct behavior.

One design point carried over from the earlier automated review that's still unresolved and worth a follow-up: IsTUnitIsNotNullMethod hardcodes the three known bases (CollectionAssertionBase, AsyncEnumerableAssertionBase, AsyncDelegateAssertion) via GetTypeByMetadataName. Thats the same shape of bug that caused this regression (#6700 hardcoded only AssertionExtensions.IsNotNull) — the next new source type that hides IsNotNull/IsNull for return-type narrowing (e.g. a future FrozenSet/ImmutableList assertion base) will silently fall through to "not suppressed" again unless someone remembers to extend this list. A marker (e.g. [SuppressibleNullCheck] on each declared IsNotNull(), or a shared internal interface those bases implement) would let the analyzer check for the marker instead of enumerating types, so adding a new base type "just works" without an analyzer change. Not a blocker given current coverage is complete and tested, but worth a fast-follow issue so this doesnt recur a third time.

No other issues found. Tests are thorough and the regression scope (collections, async, chain-safety, source-lookalikes) matches the bug report.