fix(mocks): omit 'struct' constraint when type parameter has 'unmanaged' constraint by thomhurst · Pull Request #6473 · thomhurst/TUnit

thomhurst · 2026-07-23T00:58:54Z

Summary

Mock.Of<T>() over a generic interface whose type parameter has an unmanaged constraint generated code that does not compile. Roslyn reports HasValueTypeConstraint == true for unmanaged-constrained type parameters, so GetGenericConstraints emitted where T : struct, unmanaged — invalid C# (CS0449), cascading into ~30 CS8377 errors across the generated mock, impl, factory, and extension files.

Fix

In MethodSymbolExtensions.GetGenericConstraints (the single constraint-emission helper, used for both interface-level and method-level type parameters), only emit struct when the type parameter does not also have the unmanaged constraint. unmanaged already implies a non-nullable value type.

Tests

New snapshot test Interface_With_Unmanaged_Constrained_Type_Parameter covering where T : unmanaged (with the issue's ref T repro shape) and where T : unmanaged, IDisposable.
New compile-guard test Interface_With_Unmanaged_Constrained_Type_Parameter_Compiles asserting the generated output produces no CS0449/CS8377.
Updated Interface_With_Generic_Method_Constraints_On_Explicit_Impl.verified.txt: the pre-existing GetUnmanaged<T>() case now emits where T : unmanaged instead of where T : struct, unmanaged (only change in the snapshot).

All 80 TUnit.Mocks.SourceGenerator.Tests and 1160 TUnit.Mocks.Tests pass locally (net10.0).

…ed' constraint Roslyn reports HasValueTypeConstraint for 'unmanaged'-constrained type parameters, so the mock generator emitted 'where T : struct, unmanaged', which is invalid C# (CS0449) and cascades into CS8377 across the generated files. Emit only 'unmanaged', which already implies a non-nullable value type. Fixes #6471

greptile-apps · 2026-07-23T01:01:14Z

Greptile Summary

This PR fixes a bug where Mock.Of<T>() over interfaces with an unmanaged type parameter constraint generated invalid C# (where T : struct, unmanaged), because Roslyn sets HasValueTypeConstraint = true for unmanaged-constrained parameters and the generator was emitting both. The one-line fix in GetGenericConstraints gates the struct emission behind !HasUnmanagedTypeConstraint.

Core fix (MethodSymbolExtensions.cs): Adds a && !typeParam.HasUnmanagedTypeConstraint guard so struct is suppressed when unmanaged is already being emitted, eliminating CS0449 and its ~30 cascading CS8377 errors.
Tests: Adds a snapshot test covering where T : unmanaged and where T : unmanaged, IDisposable, plus a compile-guard test that explicitly asserts CS0449/CS8377 are absent from the generated output.
Snapshot update: Interface_With_Generic_Method_Constraints_On_Explicit_Impl.verified.txt corrected for the pre-existing GetUnmanaged<T>() case (only change is removing the erroneous struct,).

Confidence Score: 5/5

Safe to merge — the change is a targeted one-line guard that eliminates invalid generated code without affecting any other constraint path.

The fix is minimal and precisely scoped, targeting only the struct emission branch without touching any other constraint path. The existing IsUnconstrained helper already handles HasUnmanagedTypeConstraint independently, keeping the change internally consistent. Two new tests (snapshot + compile-guard) and an updated pre-existing snapshot directly verify the fix eliminates the invalid constraint combination.

No files require special attention.

Important Files Changed

Filename	Overview
src/TUnit.Mocks.SourceGenerator/Extensions/MethodSymbolExtensions.cs	Single-line guard added to suppress `struct` when `unmanaged` is already emitted; fix is minimal, correct, and matches C# language rules.
tests/TUnit.Mocks.SourceGenerator.Tests/MockGeneratorTests.cs	Two new tests added: snapshot test covering both `unmanaged` and `unmanaged, IDisposable` interfaces, and a compile-guard test asserting CS0449/CS8377 are absent from generated output.
tests/TUnit.Mocks.SourceGenerator.Tests/Snapshots/Interface_With_Unmanaged_Constrained_Type_Parameter.verified.txt	New snapshot showing correct `where T : unmanaged` constraint (without `struct,`) across all generated files (mock, impl, factory, extensions).
tests/TUnit.Mocks.SourceGenerator.Tests/Snapshots/Interface_With_Generic_Method_Constraints_On_Explicit_Impl.verified.txt	Two lines updated to remove the erroneous `struct,` prefix from `GetUnmanaged<T>()` constraint in the existing snapshot — correctly reflects the fixed generator output.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[ITypeParameterSymbol] --> B{HasReferenceTypeConstraint?}
    B -- Yes --> C[emit 'class']
    B -- No --> D{HasValueTypeConstraint?}
    C --> D
    D -- No --> E{HasUnmanagedTypeConstraint?}
    D -- Yes --> F{HasUnmanagedTypeConstraint?}
    F -- No --> G[emit 'struct']
    F -- Yes --> H["skip 'struct' fix #6471"]
    G --> E
    H --> E
    E -- Yes --> I[emit 'unmanaged']
    E -- No --> J{HasNotNullConstraint?}
    I --> J
    J -- Yes --> K[emit 'notnull']
    J -- No --> L[emit ConstraintTypes]
    K --> L
    L --> M{HasConstructorConstraint?}
    M -- Yes --> N["emit 'new()'"]
    M -- No --> O["join with comma"]
    N --> O

_{Reviews (1): Last reviewed commit: "fix(mocks): omit 'struct' constraint whe..." | Re-trigger Greptile}

claude · 2026-07-23T01:03:58Z

Code review

The fix itself is correct, minimal, and well-tested: unmanaged implies HasValueTypeConstraint == true in Roslyn, so unconditionally emitting struct alongside it produced invalid where T : struct, unmanaged (CS0449). Gating struct emission on !HasUnmanagedTypeConstraint in

TUnit/src/TUnit.Mocks.SourceGenerator/Extensions/MethodSymbolExtensions.cs

Lines 36 to 44 in 8a71f00

    
           if (typeParam.HasReferenceTypeConstraint) 
        
               constraints.Add("class"); 
        
           // Roslyn reports HasValueTypeConstraint for 'unmanaged' too, but C# forbids 
        
           // combining 'struct' with 'unmanaged' (CS0449) — emit only 'unmanaged' (#6471) 
        
           if (typeParam.HasValueTypeConstraint && !typeParam.HasUnmanagedTypeConstraint) 
        
               constraints.Add("struct"); 
        
           if (typeParam.HasUnmanagedTypeConstraint) 
        
               constraints.Add("unmanaged");

is the right fix, and the new snapshot + compile-guard tests (plus the corrected pre-existing snapshot) directly cover it.

Suggestion: the same bug pattern is duplicated — and still live — elsewhere

GetGenericConstraints isn't the only place in the repo that builds a where clause from ITypeParameterSymbol. At least one sibling generator has the exact same bug this PR fixes:

TUnit/src/TUnit.Assertions.SourceGenerator/Generators/CollectionShapeAssertionGenerator.cs

Lines 261 to 282 in 8a71f00

    
           private static string? Constraint(ITypeParameterSymbol tp) 
        
           { 
        
               var cs = new List<string>(); 
        
               if (tp.HasReferenceTypeConstraint) 
        
               { 
        
                   cs.Add("class"); 
        
               } 
        
               if (tp.HasValueTypeConstraint) 
        
               { 
        
                   cs.Add("struct"); 
        
               } 
        
               if (tp.HasNotNullConstraint) 
        
               { 
        
                   cs.Add("notnull"); 
        
               } 
        
               if (tp.HasUnmanagedTypeConstraint) 
        
               { 
        
                   cs.Add("unmanaged"); 
        
               }

Constraint(tp) adds "struct" whenever HasValueTypeConstraint and separately adds "unmanaged" whenever HasUnmanagedTypeConstraint — same unguarded pair of ifs, same CS0449 for a forwarded collection-assertion method whose type parameter is unmanaged-constrained. This is reachable from RenderMethod (used to forward LINQ-shaped assertion extension methods), not dead code.

A related but distinct issue: PropertyInjectionSourceGenerator.cs (src/TUnit.Core.SourceGenerator/Generators/PropertyInjectionSourceGenerator.cs:1263) builds constraints with an else if chain across HasValueTypeConstraint / HasNotNullConstraint / HasUnmanagedTypeConstraint. Since unmanaged also sets HasValueTypeConstraint, the struct branch wins and the unmanaged branch is unreachable for those type parameters — so instead of a compile error, it silently downgrades an unmanaged constraint to struct in generated property-injection code.

Why this matters beyond "another instance to patch": the constraint-string-building logic (class / struct vs unmanaged / notnull / constraint types / new()) is copy-pasted across TUnit.Mocks.SourceGenerator, TUnit.Core.SourceGenerator, and TUnit.Assertions.SourceGenerator — I count 6+ near-identical implementations. That's why the same Roslyn quirk (unmanaged ⟹ HasValueTypeConstraint) got fixed once here but is still latent in siblings. Since these generator projects don't currently share a common library, consider extracting a single GetGenericConstraints-style helper (e.g. into a shared TUnit.*.SourceGenerator.Common-style project, if one doesn't already exist) that every generator's where-clause construction funnels through. That turns "remember to fix this in N places" into "fix it once," and a single set of tests could cover the constraint-emission logic for every constraint combination instead of re-deriving it per generator.

This is out of scope for this specific PR (which is correctly scoped to the reported issue in the mocks generator), so not a blocker — but worth a fast follow-up before someone hits CS0449 again via CollectionShapeAssertionGenerator, and worth keeping in mind as a target for the consolidation.

thomhurst · 2026-07-23T04:24:13Z

Follow-up for the review comment above (sibling generators with the same unmanaged constraint bug): #6474 — fixes CollectionShapeAssertionGenerator (CS0449), PropertyInjectionSourceGenerator (silent downgrade), and the four emitters that dropped unmanaged entirely. Shared-helper consolidation left as a separate refactor.

…6474) Follow-up to #6473 (review feedback): the same Roslyn quirk — 'unmanaged' also sets HasValueTypeConstraint — was latent in sibling generators: - CollectionShapeAssertionGenerator: emitted 'where T : struct, unmanaged' (CS0449) for forwarded methods with an 'unmanaged' type parameter. - PropertyInjectionSourceGenerator: 'struct' branch of the else-if chain shadowed the 'unmanaged' branch, silently downgrading the constraint. - AssertionExtensionGenerator, AssertionMethodGenerator, MethodAssertionGenerator, ShouldExtensionGenerator: never emitted 'unmanaged' at all, downgrading it to 'struct' (CS8377 when the generated code forwards to the unmanaged-constrained original). All sites now emit 'struct' only when the type parameter is not 'unmanaged'-constrained, and emit 'unmanaged' where it was missing. Refs #6471

Updated [TUnit](https://github.com/thomhurst/TUnit) from 1.61.15 to 1.61.38. <details> <summary>Release notes</summary> _Sourced from [TUnit's releases](https://github.com/thomhurst/TUnit/releases)._ ## 1.61.38  ## What's Changed ### Other Changes * fix: apply 'unmanaged' constraint handling to remaining source generators by @thomhurst in thomhurst/TUnit#6474 * feat: expose data source attributes on TestContext by @thomhurst in thomhurst/TUnit#6477 ### Dependencies * chore(deps): update tunit to 1.61.35 by @thomhurst in thomhurst/TUnit#6475 **Full Changelog**: thomhurst/TUnit@v1.61.35...v1.61.38 ## 1.61.35  ## What's Changed ### Other Changes * fix(mocks): omit 'struct' constraint when type parameter has 'unmanaged' constraint by @thomhurst in thomhurst/TUnit#6473 ### Dependencies * chore(deps): bump fast-uri from 3.1.2 to 3.1.4 in /docs by @dependabot[bot] in thomhurst/TUnit#6466 * chore(deps): update tunit to 1.61.29 by @thomhurst in thomhurst/TUnit#6467 * chore(deps): update dependency verify.tool to v0.8.0 by @thomhurst in thomhurst/TUnit#6470 * chore(deps): update dependency docusaurus-plugin-llms to ^0.5.1 by @thomhurst in thomhurst/TUnit#6472 **Full Changelog**: thomhurst/TUnit@v1.61.29...v1.61.35 ## 1.61.29  ## What's Changed ### Other Changes * perf: remove EnumerableAsyncProcessor dependency by @thomhurst in thomhurst/TUnit#6465 ### Dependencies * chore(deps): bump shell-quote from 1.8.4 to 1.10.0 in /docs by @dependabot[bot] in thomhurst/TUnit#6462 * chore(deps): bump body-parser from 1.20.5 to 1.20.6 in /docs by @dependabot[bot] in thomhurst/TUnit#6461 * chore(deps): bump webpack-dev-server from 5.2.5 to 5.2.6 in /docs by @dependabot[bot] in thomhurst/TUnit#6460 * chore(deps): update dependency microsoft.kiota.abstractions to v2 by @thomhurst in thomhurst/TUnit#6464 * chore(deps): update tunit to 1.61.23 by @thomhurst in thomhurst/TUnit#6463 **Full Changelog**: thomhurst/TUnit@v1.61.23...v1.61.29 ## 1.61.23  ## What's Changed ### Other Changes * fix(mocks): emit 'where T : default' for interface-constrained generic methods by @thomhurst in thomhurst/TUnit#6458 ### Dependencies * chore(deps): update tunit to 1.61.15 by @thomhurst in thomhurst/TUnit#6450 * chore(deps): update actions/checkout action to v7.0.1 by @thomhurst in thomhurst/TUnit#6453 * chore(deps): update verify to 31.26.0 by @thomhurst in thomhurst/TUnit#6454 * chore(deps): update verify to 31.27.0 by @thomhurst in thomhurst/TUnit#6457 * chore(deps): update react to ^19.2.8 by @thomhurst in thomhurst/TUnit#6459 **Full Changelog**: thomhurst/TUnit@v1.61.15...v1.61.23 Commits viewable in [compare view](thomhurst/TUnit@v1.61.15...v1.61.38). </details> [![Dependabot compatibility score](https://dependabot-badges.githubapp.com/badges/compatibility_score?dependency-name=TUnit&package-manager=nuget&previous-version=1.61.15&new-version=1.61.38)](https://docs.github.com/en/github/managing-security-vulnerabilities/about-dependabot-security-updates#about-compatibility-scores) Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting `@dependabot rebase`. [//]: # (dependabot-automerge-start) [//]: # (dependabot-automerge-end) --- <details> <summary>Dependabot commands and options</summary> <br /> You can trigger Dependabot actions by commenting on this PR: - `@dependabot rebase` will rebase this PR - `@dependabot recreate` will recreate this PR, overwriting any edits that have been made to it - `@dependabot show <dependency name> ignore conditions` will show all of the ignore conditions of the specified dependency - `@dependabot ignore this major version` will close this PR and stop Dependabot creating any more for this major version (unless you reopen the PR or upgrade to it yourself) - `@dependabot ignore this minor version` will close this PR and stop Dependabot creating any more for this minor version (unless you reopen the PR or upgrade to it yourself) - `@dependabot ignore this dependency` will close this PR and stop Dependabot creating any more for this dependency (unless you reopen the PR or upgrade to it yourself) </details> Signed-off-by: dependabot[bot] <support@github.com> Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Updated [TUnit.Core](https://github.com/thomhurst/TUnit) from 1.61.15 to 1.61.38. <details> <summary>Release notes</summary> _Sourced from [TUnit.Core's releases](https://github.com/thomhurst/TUnit/releases)._ ## 1.61.38  ## What's Changed ### Other Changes * fix: apply 'unmanaged' constraint handling to remaining source generators by @thomhurst in thomhurst/TUnit#6474 * feat: expose data source attributes on TestContext by @thomhurst in thomhurst/TUnit#6477 ### Dependencies * chore(deps): update tunit to 1.61.35 by @thomhurst in thomhurst/TUnit#6475 **Full Changelog**: thomhurst/TUnit@v1.61.35...v1.61.38 ## 1.61.35  ## What's Changed ### Other Changes * fix(mocks): omit 'struct' constraint when type parameter has 'unmanaged' constraint by @thomhurst in thomhurst/TUnit#6473 ### Dependencies * chore(deps): bump fast-uri from 3.1.2 to 3.1.4 in /docs by @dependabot[bot] in thomhurst/TUnit#6466 * chore(deps): update tunit to 1.61.29 by @thomhurst in thomhurst/TUnit#6467 * chore(deps): update dependency verify.tool to v0.8.0 by @thomhurst in thomhurst/TUnit#6470 * chore(deps): update dependency docusaurus-plugin-llms to ^0.5.1 by @thomhurst in thomhurst/TUnit#6472 **Full Changelog**: thomhurst/TUnit@v1.61.29...v1.61.35 ## 1.61.29  ## What's Changed ### Other Changes * perf: remove EnumerableAsyncProcessor dependency by @thomhurst in thomhurst/TUnit#6465 ### Dependencies * chore(deps): bump shell-quote from 1.8.4 to 1.10.0 in /docs by @dependabot[bot] in thomhurst/TUnit#6462 * chore(deps): bump body-parser from 1.20.5 to 1.20.6 in /docs by @dependabot[bot] in thomhurst/TUnit#6461 * chore(deps): bump webpack-dev-server from 5.2.5 to 5.2.6 in /docs by @dependabot[bot] in thomhurst/TUnit#6460 * chore(deps): update dependency microsoft.kiota.abstractions to v2 by @thomhurst in thomhurst/TUnit#6464 * chore(deps): update tunit to 1.61.23 by @thomhurst in thomhurst/TUnit#6463 **Full Changelog**: thomhurst/TUnit@v1.61.23...v1.61.29 ## 1.61.23  ## What's Changed ### Other Changes * fix(mocks): emit 'where T : default' for interface-constrained generic methods by @thomhurst in thomhurst/TUnit#6458 ### Dependencies * chore(deps): update tunit to 1.61.15 by @thomhurst in thomhurst/TUnit#6450 * chore(deps): update actions/checkout action to v7.0.1 by @thomhurst in thomhurst/TUnit#6453 * chore(deps): update verify to 31.26.0 by @thomhurst in thomhurst/TUnit#6454 * chore(deps): update verify to 31.27.0 by @thomhurst in thomhurst/TUnit#6457 * chore(deps): update react to ^19.2.8 by @thomhurst in thomhurst/TUnit#6459 **Full Changelog**: thomhurst/TUnit@v1.61.15...v1.61.23 Commits viewable in [compare view](thomhurst/TUnit@v1.61.15...v1.61.38). </details> [![Dependabot compatibility score](https://dependabot-badges.githubapp.com/badges/compatibility_score?dependency-name=TUnit.Core&package-manager=nuget&previous-version=1.61.15&new-version=1.61.38)](https://docs.github.com/en/github/managing-security-vulnerabilities/about-dependabot-security-updates#about-compatibility-scores) Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting `@dependabot rebase`. [//]: # (dependabot-automerge-start) [//]: # (dependabot-automerge-end) --- <details> <summary>Dependabot commands and options</summary> <br /> You can trigger Dependabot actions by commenting on this PR: - `@dependabot rebase` will rebase this PR - `@dependabot recreate` will recreate this PR, overwriting any edits that have been made to it - `@dependabot show <dependency name> ignore conditions` will show all of the ignore conditions of the specified dependency - `@dependabot ignore this major version` will close this PR and stop Dependabot creating any more for this major version (unless you reopen the PR or upgrade to it yourself) - `@dependabot ignore this minor version` will close this PR and stop Dependabot creating any more for this minor version (unless you reopen the PR or upgrade to it yourself) - `@dependabot ignore this dependency` will close this PR and stop Dependabot creating any more for this dependency (unless you reopen the PR or upgrade to it yourself) </details> Signed-off-by: dependabot[bot] <support@github.com> Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

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fix: apply 'unmanaged' constraint handling to remaining source generators #6474

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fix(mocks): omit 'struct' constraint when type parameter has 'unmanaged' constraint - #6473

fix(mocks): omit 'struct' constraint when type parameter has 'unmanaged' constraint#6473
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fix/6471-unmanaged-constraint

thomhurst commented Jul 23, 2026

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thomhurst commented Jul 23, 2026

Summary

Fix

Tests

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greptile-apps Bot commented Jul 23, 2026

Greptile Summary

Confidence Score: 5/5

Important Files Changed

Flowchart

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claude Bot commented Jul 23, 2026

Code review

Suggestion: the same bug pattern is duplicated — and still live — elsewhere

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thomhurst commented Jul 23, 2026

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