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::`, clash-free with user member names, valid under nullable and warnings-as-errors, and deterministic. The generator reports no diagnostics: it skips what it cannot mock and `MockabilityAnalyzer` reports it, so mockability rules change in both. Keep the generator incremental: pipeline entities must have value equality (`EquatableArray`, records) so caching works; `MockGenerator.EqualityTests` covers this.
 - **Arity**: the library hand-writes setup and verification types for up to four parameters; the generator emits the same shapes for more. A change to one arity applies to every arity in both places.

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/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` under `LangVersion` preview) 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
@@ -6,7 +6,7 @@ description: Mockolate is a source-generated, strongly-typed and AOT-compatible
 
 <img align="right" width="200" src="https://raw.githubusercontent.com/Testably/Mockolate/main/Docs/Mockolate_256x256.png" alt="Mockolate logo" />
 
-[**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.
+[**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, .NET 11 and .NET Framework 4.8.
 
 ```csharp
 IChocolateDispenser dispenser = IChocolateDispenser.CreateMock();

diff --git a/Docs/pages/05-setup/01-methods.md b/Docs/pages/05-setup/01-methods.md
@@ -4,8 +4,7 @@ description: Set up return values, exceptions and callbacks for methods of a Moc
 
 # Methods
 
-Use `dispenser.Mock.Setup.MethodName(…)` to set up a method. Pass a value or a
-[parameter matcher](../06-parameter-matching.md) for each parameter to decide which calls the setup applies to.
+Use `dispenser.Mock.Setup.MethodName(…)` to set up a method. Pass a value or a [parameter matcher](../06-parameter-matching.md) for each parameter to decide which calls the setup applies to. With C# 15 you can also pass a predicate directly (see [Union parameters](../06-parameter-matching.md#union-parameters-c-15)).
 
 ## Returns / Throws
 

diff --git a/Docs/pages/05-setup/03-indexers.md b/Docs/pages/05-setup/03-indexers.md
@@ -78,6 +78,18 @@ When several setups match the same key, the one defined last wins.
 Use `.SkippingBaseClass()` to prevent calls to the base class implementation for a single indexer, or
 [`MockBehavior.SkippingBaseClass()`](../04-create-mocks.md#customizing-mock-behavior) for the whole mock.
 
+## Union parameters (C# 15)
+
+With C# 15, indexer keys accept predicates directly, in setups and verifications alike:
+
+```csharp
+dispenser.Mock.Setup[type => type.StartsWith("D")].Returns(10);
+
+dispenser.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](../06-parameter-matching.md#union-parameters-c-15) for the details.
+
 ## Indexers with only one accessor
 
 As for [properties](02-properties.md#properties-with-only-one-accessor), the setup and verify members only offer the