vbreuss · Sep 4, 2026 · Sep 4, 2026 · Sep 4, 2026 · Sep 4, 2026 · Sep 5, 2026
diff --git a/.fallout/build.schema.json b/.fallout/build.schema.json
@@ -44,6 +44,7 @@
         "Pack",
         "PublishBenchmarkReport",
         "Restore",
+        "UnionTests",
         "UnitTests",
         "UpdateReadme"
       ]

diff --git a/.gitattributes b/.gitattributes
@@ -13,3 +13,6 @@
 # while mounting and running tests in Linux
 *.sh    text eol=lf
 build   text eol=lf
+
+# Snapshot files are regenerated by AcceptSnapshotChanges; collapse them in GitHub diffs
+Tests/Mockolate.SourceGenerators.Tests/Snapshot/Expected/** linguist-generated=true
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
@@ -28,6 +28,7 @@ jobs:
                     dotnet-version: |
                         8.0.x
                         10.0.x
+                    global-json-file: global.json
             -   name: Run unit tests (windows)
                 if: matrix.os == 'windows-latest'
                 run: ./build.ps1 CodeCoverage
@@ -60,6 +61,7 @@ jobs:
                     dotnet-version: |
                         8.0.x
                         10.0.x
+                    global-json-file: global.json
             -   name: Run AOT smoke tests (windows)
                 if: matrix.os == 'windows-latest'
                 run: ./build.ps1 AotSmokeTests
@@ -82,6 +84,7 @@ jobs:
                     dotnet-version: |
                         8.0.x
                         10.0.x
+                    global-json-file: global.json
             -   name: API checks
                 run: ./build.sh ApiChecks
             -   name: Upload artifacts
@@ -93,6 +96,33 @@ jobs:
                         ./Artifacts/*
                         ./TestResults/*.trx
 
+    union-tests:
+        name: "Union tests (net11.0, C# preview)"
+        runs-on: ubuntu-latest
+        env:
+            DOTNET_NOLOGO: true
+        steps:
+            -   uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+                with:
+                    fetch-depth: 0
+            -   name: Setup .NET SDKs
+                uses: actions/setup-dotnet@a98b56852c35b8e3190ac28c8c2271da59106c68 # v6.0.0
+                with:
+                    dotnet-version: |
+                        8.0.x
+                        10.0.x
+                    global-json-file: global.json
+            -   name: Union tests
+                run: ./build.sh UnionTests
+            -   name: Upload artifacts
+                if: always()
+                uses: actions/upload-artifact@cf430e030ddbb5b0abf93d22962f4752f3646cd9 # v7.0.2
+                with:
+                    name: Union-tests
+                    path: |
+                        ./Artifacts/*
+                        ./TestResults/*.trx
+
     benchmarks:
         name: "Benchmarks (${{ matrix.benchmark }})"
         strategy:
@@ -120,6 +150,7 @@ jobs:
                     dotnet-version: |
                         8.0.x
                         10.0.x
+                    global-json-file: global.json
             -   name: Run benchmarks
                 run: ./build.sh Benchmarks --benchmark-filter "*${{ matrix.benchmark }}*"
             -   name: Upload artifacts
@@ -150,6 +181,7 @@ jobs:
                 with:
                     dotnet-version: |
                         10.0.x
+                    global-json-file: global.json
             -   name: Publish benchmark report
                 run: ./build.sh PublishBenchmarkReport
                 env:
@@ -172,6 +204,7 @@ jobs:
                     dotnet-version: |
                         8.0.x
                         10.0.x
+                    global-json-file: global.json
             -   name: Run mutation tests
                 run: ./build.sh MutationTests MutationTestsDashboard
 
@@ -192,12 +225,13 @@ jobs:
                     dotnet-version: |
                         8.0.x
                         10.0.x
+                    global-json-file: global.json
             -   name: Run sonarcloud analysis
                 run: ./build.sh CodeAnalysis
 
     publish-test-results:
         name: "Publish Tests Results"
-        needs: [ api-tests, unit-tests, aot-smoke-tests ]
+        needs: [ api-tests, unit-tests, aot-smoke-tests, union-tests ]
         runs-on: ubuntu-latest
         permissions:
             checks: write
@@ -229,6 +263,7 @@ jobs:
                     dotnet-version: |
                         8.0.x
                         10.0.x
+                    global-json-file: global.json
             -   name: Pack nuget packages
                 run: ./build.sh Pack
             -   name: Upload packages
@@ -243,7 +278,7 @@ jobs:
         if: ${{ startsWith(github.ref, 'refs/tags/v') }}
         runs-on: macos-latest
         environment: production
-        needs: [ pack, unit-tests, aot-smoke-tests, api-tests, publish-test-results, static-code-analysis ]
+        needs: [ pack, unit-tests, aot-smoke-tests, api-tests, union-tests, publish-test-results, static-code-analysis ]
         permissions:
             contents: write
             id-token: write

diff --git a/.github/workflows/ci-analysis.yml b/.github/workflows/ci-analysis.yml
@@ -24,6 +24,7 @@ jobs:
                 with:
                     dotnet-version: |
                         10.0.x
+                    global-json-file: global.json
             -   name: Upload mutation dashboard and create comment
                 run: ./build.sh MutationTestsComment
                 # The artifacts of the workflow run are untrusted, because a pull request controls the code that
@@ -53,6 +54,7 @@ jobs:
                 with:
                     dotnet-version: |
                         10.0.x
+                    global-json-file: global.json
             -   name: Create benchmark comment
                 run: ./build.sh BenchmarkComment
                 # The artifacts of the workflow run are untrusted, because a pull request controls the code that

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
@@ -23,6 +23,7 @@ jobs:
                     dotnet-version: |
                         8.0.x
                         10.0.x
+                    global-json-file: global.json
             -   name: Run unit tests (windows)
                 if: matrix.os == 'windows-latest'
                 run: ./build.ps1 CodeCoverage
@@ -55,6 +56,7 @@ jobs:
                     dotnet-version: |
                         8.0.x
                         10.0.x
+                    global-json-file: global.json
             -   name: Run AOT smoke tests (windows)
                 if: matrix.os == 'windows-latest'
                 run: ./build.ps1 AotSmokeTests
@@ -77,6 +79,7 @@ jobs:
                     dotnet-version: |
                         8.0.x
                         10.0.x
+                    global-json-file: global.json
             -   name: API checks
                 run: ./build.sh ApiChecks
             -   name: Upload artifacts
@@ -103,6 +106,7 @@ jobs:
                     dotnet-version: |
                         8.0.x
                         10.0.x
+                    global-json-file: global.json
             -   name: Run mutation tests
                 run: ./build.sh MutationTests
             -   name: Upload artifacts
@@ -113,6 +117,33 @@ jobs:
                     path: |
                         ./Artifacts/*
 
+    union-tests:
+        name: "Union tests (net11.0, C# preview)"
+        runs-on: ubuntu-latest
+        env:
+            DOTNET_NOLOGO: true
+        steps:
+            -   uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+                with:
+                    fetch-depth: 0
+            -   name: Setup .NET SDKs
+                uses: actions/setup-dotnet@a98b56852c35b8e3190ac28c8c2271da59106c68 # v6.0.0
+                with:
+                    dotnet-version: |
+                        8.0.x
+                        10.0.x
+                    global-json-file: global.json
+            -   name: Union tests
+                run: ./build.sh UnionTests
+            -   name: Upload artifacts
+                if: always()
+                uses: actions/upload-artifact@cf430e030ddbb5b0abf93d22962f4752f3646cd9 # v7.0.2
+                with:
+                    name: Union-tests
+                    path: |
+                        ./Artifacts/*
+                        ./TestResults/*.trx
+
     benchmarks:
         name: "Benchmarks (${{ matrix.benchmark }})"
         strategy:
@@ -138,6 +169,7 @@ jobs:
                     dotnet-version: |
                         8.0.x
                         10.0.x
+                    global-json-file: global.json
             -   name: Run benchmarks
                 run: ./build.sh Benchmarks --benchmark-filter "*${{ matrix.benchmark }}*"
             -   name: Upload artifacts
@@ -167,12 +199,13 @@ jobs:
                     dotnet-version: |
                         8.0.x
                         10.0.x
+                    global-json-file: global.json
             -   name: Run sonarcloud analysis
                 run: ./build.sh CodeAnalysis
 
     publish-test-results:
         name: "Publish Tests Results"
-        needs: [ api-tests, unit-tests, aot-smoke-tests ]
+        needs: [ api-tests, unit-tests, aot-smoke-tests, union-tests ]
         runs-on: ubuntu-latest
         permissions:
             checks: write

diff --git a/AGENTS.md b/AGENTS.md
@@ -4,7 +4,7 @@ Mockolate is a source-generated mocking library for .NET. Users write `IFoo.Crea
 
 ## SDK and Build
 
-- Use the SDK pinned in `global.json` (.NET 10, `rollForward: latestPatch`). Tests also run on .NET 8, and on .NET Framework 4.8 on Windows only.
+- Use the SDK pinned in `global.json` (currently a .NET 11 preview, `rollForward: latestPatch`). Tests also run on .NET 10 and .NET 8, and on .NET Framework 4.8 on Windows only. On .NET 11 the test projects compile with the preview language version and set `MockolateUnionParameters`, so the union-typed setup surface is exercised there.
 - Build from the root with `dotnet build Mockolate.slnx`. Shared settings live in the root `Directory.Build.props` (strong naming, `MinimumRoslynVersion`), `Source/Directory.Build.props` (target frameworks, warnings as errors, AOT and trimming flags) and `Tests/Directory.Build.props` (test frameworks, TUnit, aweXpect).
 - Package versions belong only in `Directory.Packages.props`. `VersionOverride` is used only to pin Roslyn packages to `$(MinimumRoslynVersion)`.
 
@@ -14,7 +14,7 @@ Correctness is a must. The other goals are strong defaults and win in this order
 
 ## Project Constraints
 
-- **Target frameworks**: `Mockolate` ships `netstandard2.0`, `net8.0` and `net10.0`. The generator, analyzers and code fixers target `netstandard2.0` only. Code compiled for `netstandard2.0` or `net48` has no `IsExternalInit`, `System.Index` or `System.Range` unless polyfilled, so records, `init` and list patterns need care: `Mockolate` polyfills `IsExternalInit`, `Mockolate.Tests` does not, and guards such code with `#if NET8_0_OR_GREATER`.
+- **Target frameworks**: `Mockolate` ships `netstandard2.0`, `net8.0`, `net10.0` and `net11.0`. The generator, analyzers and code fixers target `netstandard2.0` only. Code compiled for `netstandard2.0` or `net48` has no `IsExternalInit`, `System.Index` or `System.Range` unless polyfilled, so records, `init` and list patterns need care: `Mockolate` polyfills `IsExternalInit`, `Mockolate.Tests` does not, and guards such code with `#if NET8_0_OR_GREATER`.
 - **Roslyn version**: the generator, analyzers and code fixers compile against `MinimumRoslynVersion` (5.0.0), the oldest compiler they must load in. Never use a newer Roslyn API. It cannot drop below 5.0 because the emitted `Mock.g.cs` uses a C# 14 extension block.
 - **Generated code** runs inside every consumer's compilation. Keep it reflection-free, fully qualified with `global::`, valid under nullable and warnings-as-errors, and deterministic. Keep the generator incremental: pipeline entities must have value equality (`EquatableArray`, records) so caching works; `MockGenerator.EqualityTests` covers this.
 - **Analyzers**: rule IDs are `MockolateNNNN`. Add the descriptor in `Rules.cs`, its texts in `Resources.resx`, the row in `AnalyzerReleases.Unshipped.md` and the user docs in `Docs/pages/08-analyzers.md`.

diff --git a/Benchmarks/Mockolate.Benchmarks/Mockolate.Benchmarks.csproj b/Benchmarks/Mockolate.Benchmarks/Mockolate.Benchmarks.csproj
@@ -1,8 +1,12 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
+	<!-- Pin the classic overloads so results stay comparable to the CI baseline once the SDK defaults to C# 15. -->
+	<Import Project="..\..\Source\Mockolate\build\Mockolate.props"/>
+
 	<PropertyGroup>
 		<OutputType>Exe</OutputType>
-		<TargetFramework>net10.0</TargetFramework>
+		<TargetFramework>net11.0</TargetFramework>
+		<MockolateUnionParameters>false</MockolateUnionParameters>
 		<ImplicitUsings>enable</ImplicitUsings>
 		<Nullable>enable</Nullable>
 		<GenerateProgramFile>false</GenerateProgramFile>

diff --git a/Directory.Packages.props b/Directory.Packages.props
@@ -32,7 +32,7 @@
 		<PackageVersion Include="TUnit.Engine" Version="1.75.0"/>
 		<PackageVersion Include="Microsoft.Testing.Extensions.TrxReport" Version="2.5.1"/>
 		<PackageVersion Include="Microsoft.Testing.Extensions.CodeCoverage" Version="18.12.0"/>
-		<PackageVersion Include="aweXpect.Snapshots.PublicApi" Version="0.5.0"/>
+		<PackageVersion Include="aweXpect.Snapshots.PublicApi" Version="0.6.0"/>
 		<PackageVersion Include="aweXpect" Version="3.4.0"/>
 		<PackageVersion Include="aweXpect.Chronology" Version="1.1.0"/>
 		<PackageVersion Include="aweXpect.Mockolate" Version="3.4.0"/>

diff --git a/Docs/agents/project-structure.md b/Docs/agents/project-structure.md
@@ -18,7 +18,7 @@ Only the `Mockolate` package is published. It carries the generator, analyzers a
 
 ### `Source/Mockolate`
 
-The runtime library (`netstandard2.0`, `net8.0`, `net10.0`).
+The runtime library (`netstandard2.0`, `net8.0`, `net10.0`, `net11.0`).
 
 | Location | Responsibility |
 |---|---|
@@ -43,6 +43,7 @@ An `IIncrementalGenerator` (`MockGenerator.cs`, `netstandard2.0`).
 - `Entities/` holds the equatable model extracted from symbols (`Class`, `Method`, `Property`, `Event`, `EquatableArray`, ...); `Internals/EntityCache.cs` deduplicates it per compilation.
 - `Sources/` turns the model into C# with `StringBuilder`.
 - Output depends on the consumer's compilation, for example whether `OverloadResolutionPriorityAttribute` exists.
+- Union mode (`Sources.MockClass.Unions.cs`): with C# 15 (or `MockolateUnionParameters=true`) the setup and verify overloads take `ParameterArg<T>?` or `Func<T, bool>` per parameter instead of the classic matcher and value set; below that the output is unchanged.
 
 ### `Source/Mockolate.Analyzers` and `Source/Mockolate.Analyzers.CodeFixers`
 

diff --git a/Docs/agents/workflows.md b/Docs/agents/workflows.md
@@ -32,8 +32,9 @@ Target frameworks per test project:
 
 | Project | Frameworks |
 |---|---|
-| `Mockolate.Tests`, `Mockolate.Internal.Tests`, `Mockolate.ExampleTests` | `net10.0`, `net8.0`, `net48` (Windows only) |
-| `Mockolate.SourceGenerators.Tests`, `Mockolate.Analyzers.Tests`, `Build.Tests` | `net10.0` |
+| `Mockolate.Tests`, `Mockolate.Internal.Tests`, `Mockolate.ExampleTests` | `net11.0` (C# preview, union parameters), `net10.0`, `net8.0`, `net48` (Windows only) |
+| `Mockolate.SourceGenerators.Tests`, `Mockolate.Analyzers.Tests` | `net11.0` |
+| `Build.Tests` | `net10.0` |
 | `Mockolate.Api.Tests` | `net8.0` |
 
 A newer language feature can compile for `net10.0` and fail for `net48`; build or test `net48` too when you use one.

diff --git a/Docs/pages/00-index.md b/Docs/pages/00-index.md
@@ -5,7 +5,7 @@
 [![Nuget](https://img.shields.io/nuget/v/Mockolate)](https://www.nuget.org/packages/Mockolate)
 
 [**Mockolate**](https://github.com/Testably/Mockolate) is a modern, strongly-typed, AOT-compatible mocking library for .NET, powered by source generators.
-It enables fast, compile-time validated mocking with .NET Standard 2.0, .NET 8, .NET 10 and .NET Framework 4.8.
+It enables fast, compile-time validated mocking with .NET Standard 2.0, .NET 8, .NET 10, .NET 11 and .NET Framework 4.8.
 
 - **Source generator-based**: No runtime proxy generation.
 - **Fast**: Direct dispatch with no reflection or dynamic proxies.

diff --git a/Docs/pages/setup/02-methods.md b/Docs/pages/setup/02-methods.md
@@ -1,6 +1,7 @@
 # Methods
 
-Use `sut.Mock.Setup.MethodName(…)` to set up methods. You can specify argument matchers for each parameter.
+Use `sut.Mock.Setup.MethodName(…)` to set up methods. You can specify argument matchers for each parameter; with C# 15
+you can also pass predicates directly (see *Union Parameters* under Parameter Matching).
 
 ## Returns / Throws
 

diff --git a/Docs/pages/setup/03-indexers.md b/Docs/pages/setup/03-indexers.md
@@ -132,3 +132,16 @@ does not intercept.
   methods.
 - Use `.SkippingBaseClass(…)` to override the base class behavior for a specific indexer (only for class mocks).
 - When you specify overlapping setups, the most recently defined setup takes precedence.
+
+## Union Parameters (C# 15)
+
+With C# 15, indexer keys accept predicates directly, in setups and verifications alike:
+
+```csharp
+sut.Mock.Setup[type => type.StartsWith("D")].Returns(10);
+
+sut.Mock.Verify[type => type.StartsWith("D")].Got().Once();
+```
+
+Only an indexer that is the sole indexer of its key count on the type takes union-typed keys; indexers sharing a key
+count keep the classic matcher/value overloads. See *Union Parameters* under Parameter Matching for the details.
diff --git a/Docs/pages/setup/04-parameter-matching.md b/Docs/pages/setup/04-parameter-matching.md
@@ -303,6 +303,44 @@ verify surface for it. A `virtual` class member is not flagged at all, because t
 to the wrapped instance or to `base` and it keeps working. See
 [Mockolate0003](../analyzers#mockolate0003) for what the member does at runtime.
 
+## Union Parameters (C# 15)
+
+When the consuming project compiles with C# 15, Mockolate generates the setup and verify overloads with union-typed
+parameters: every value-capable parameter becomes a `ParameterArg<T>?` that accepts an `It` matcher or a direct value,
+and a `Func<T, bool>` overload accepts a predicate. The number of overloads stays the same as before, but predicates no
+longer need `It.Satisfies`:
+
+```csharp
+sut.Mock.Setup.Dispense(type => type.StartsWith("D"), It.IsAny<int>()).Returns(true);
+
+sut.Mock.Verify.Dispense("Dark", amount => amount > 10).Once();
+```
+
+Union mode is enabled automatically once the compiler ships C# 15 and the project's effective language version is
+C# 15 or later. On a preview compiler, opt in with `<MockolateUnionParameters>true</MockolateUnionParameters>` in the
+project file; `false` keeps the classic overloads on any compiler.
+
+Behaviour of the union-typed parameters:
+
+- `null` and `default` stand for the parameter's declared default value, or for `default(T)` when it has none, exactly
+  as with the classic value overloads.
+- A parameter whose type is a delegate offers the raw delegate instead of a predicate, so a lambda is still matched as
+  a value.
+- For an `object` parameter, a lambda literal is a predicate, while a delegate stored in a variable is a value.
+- Within a union-typed member, `ref`, `out` and `ref readonly` parameters as well as `Span<T>` and `ReadOnlySpan<T>`
+  parameters keep their matcher slot (no predicate for that parameter); `in` parameters behave like by-value ones.
+- Overloaded method names (including a generic sibling; a same-named member in another scope — public, protected or
+  static — does not count), generic methods, `params` methods, members with custom ref-struct parameters, and indexers
+  that share their key count with another same-scope indexer keep the classic matcher/value overloads. Use
+  `It.Satisfies` for predicates there.
+
+The generated `ParameterArg<T>` type relies on `UnionAttribute`, `OverloadResolutionPriorityAttribute` and
+`CallerArgumentExpressionAttribute`. Mockolate declares whichever of them the compilation lacks (neither the referenced
+frameworks nor the project itself provide it). Another generator such as PolySharp can provide them without Mockolate
+noticing; set `<MockolateUnionAttributePolyfills>false</MockolateUnionAttributePolyfills>` to declare that
+`OverloadResolutionPriorityAttribute` and `CallerArgumentExpressionAttribute` come from elsewhere (`UnionAttribute`
+keeps following the compilation), or list the attribute names the project provides.
+
 ## Parameter Predicates
 
 When the method name is unique (no overloads), you can use argument matchers from the `Match` class for more flexible parameters matching: