spike: Metalama 2026.1 on the .NET 11 RC3 SDK

design/knowledge/KB-060-metalama-2026-1-on-the-net-11-sdk.md

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+---
+verified: 2026-10-31
+pins:
+  - external .NET-SDK 11.0.100-rc.3.26522.148
+  - external Metalama.Framework 2026.1.30
+  - external Metalama.Framework 2027.0.5-preview
+source: spikes/Sdk11Rc3Metalama, measured 2026-10-31 on macOS arm64 with the published CodoMetis.TypeKit and CodoMetis.TypeKit.Generators 2.3.1, against SDK 11.0.100-rc.3.26522.148 and 11.0.100-rc.1.26425.128, each alone in its .NET installation, and RC3 with SDK 10.0.403 beside it
+turned-on: decision 120 (a net11.0 target in 2.4.0 on Metalama 2026.1, and Metalama 2027.0 in 2.5.0), and the RC3 part of issue 170
+test: none, a measurement of SDKs and compilers that CI does not install; the spike's run.sh and web-matrix.sh repeat it
+---
+
+# KB-060. Metalama 2026.1 weaves a `net11.0` project on SDK 11 RC3 at C# 14, and a web project fails when SDK 11 is the only SDK
+
+On SDK 11.0.100-rc.3, with the 2.3.1 packages (`Metalama.Framework` 2026.1.30, `Metalama.Compiler` 2026.1.20):
+
+- A `net11.0` console project with a value object builds, weaves and runs: `From`, `Parse` and a JSON round trip.
+  No warning.
+- `#error version` names Metalama's compiler 2026.1.20 at language version 14.0. `Metalama.Framework` 2026.1.31
+  (compiler 2026.1.21) is at 14.0 too.
+- A `union` declared in the project fails with `CS0106`, `CS1001` and `CS8803`; a `switch` over a union returned by
+  a `net11.0`-only library fails with `CS8652` ("unions ... in Preview"); holding that union builds. Without Metalama
+  both compile.
+- A direct `Metalama.Framework` 2027.0.5-preview reference beside the 2.3.1 packages: compiler 2027.0.5-preview at
+  15.0, the value object woven, and both union cases build and run.
+- One project on `net10.0;net11.0` with that 2027 reference and no `LangVersion`: `net10.0` compiles at 14.0 and
+  `net11.0` at 15.0. A union under `#if NET11_0_OR_GREATER`, or in a file removed from `net10.0` by a conditional
+  `Compile Remove`, builds on both targets, and the value object is generated and runs on both. With 2026.1 both
+  targets compile at 14.0 and either union fails on `net11.0`.
+
+The first four are the RC1 results of decision 120, unchanged; decision 120 did not measure the multi-targeted project.
+
+**A web project (`Microsoft.NET.Sdk.Web`) on Metalama 2026.1 fails with `LAMA0625` when SDK 11 is the only SDK in
+its .NET installation**, on RC1 and on RC3, with 2026.1.31, and for a `net10.0` project as for a `net11.0` one. The SDK 11 Razor compiler needs Roslyn 5.11 (RC1) or
+5.12 (RC3), Metalama 2026.1 bundles 5.9, and Metalama finds no older SDK in that installation to take a compatible
+Razor compiler from ("Installed SDKs found: 11.0.100"). With SDK 10.0.403 in the same installation the build passes
+with two `LAMA0617` warnings (the ASP.NET analyzers from SDK 10.0.401, the Razor compiler from SDK 10.0.403).
+Metalama 2027.0.5-preview builds the web project with neither. Pinned to SDK 10.0.403, `net10.0`, both versions build
+it with no warning.
+
+Not measured: the general availability SDK, Linux and Windows, an IDE, a Razor Pages or Blazor project, and any
+2027 build without a Metalama license on the machine (this machine has a license entry for 2027.0).

design/knowledge/README.md

@@ -88,3 +88,4 @@ test and later for the harness's `Baseline.Fact`.
 | [KB-055](KB-055-ericksonlopez-domainprimitives.md) | EricksonLopez.DomainPrimitives 3.0.0 is an attribute on a partial record struct, with a catalogue of ready-made types, re-validated on every read | EricksonLopez.DomainPrimitives 3.0.0 | prose |
 | [KB-057](KB-057-a-nullable-contract-has-an-element-type-and-reads-null-as-null.md) | A `Nullable<T>` contract has `T` as its element type, and a JSON null read into it is null whatever `T`'s converter does | OpenApi 10.0.12, SDK 10.0.4xx | `KB057` |
 | [KB-058](KB-058-ef-orders-captured-values-in-sql-only-with-a-lambda.md) | EF orders captured values in SQL only when the chain takes a lambda: `values.Max()` runs in memory, `values.Max(v => v)` in SQL | EF Sqlite 10.0.12 | `KB058` |
+| [KB-060](KB-060-metalama-2026-1-on-the-net-11-sdk.md) | Metalama 2026.1 weaves a `net11.0` project on SDK 11 RC3 at C# 14, and a web project fails with `LAMA0625` when SDK 11 is the only SDK | .NET SDK 11 RC3, Metalama 2026.1.30, 2027.0.5-preview | prose |

spikes/Sdk11Rc3Metalama/App/App.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <!--
+      One consumer, switched per row by properties (README, "The rows"):
+        -p:ProbeVersion=true  #error version, which names the compiler and its language version
+        -p:DeclareUnion=true  a union declared here, and a switch over it
+        -p:HoldUnion=true     UnionLib's union held in a var and printed
+        -p:MatchUnion=true    a switch over UnionLib's union
+        -p:MetalamaFramework=V  a direct Metalama.Framework V reference beside the 2.3.1 packages
+        -p:NoGenerators=true  the control: no TypeKit, no Metalama, no value object
+  -->
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net11.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <UseSharedCompilation>false</UseSharedCompilation>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DefineConstants Condition="'$(ProbeVersion)' == 'true'">$(DefineConstants);PROBE_VERSION</DefineConstants>
+    <DefineConstants Condition="'$(DeclareUnion)' == 'true'">$(DefineConstants);DECLARE_UNION</DefineConstants>
+    <DefineConstants Condition="'$(HoldUnion)' == 'true'">$(DefineConstants);HOLD_UNION</DefineConstants>
+    <DefineConstants Condition="'$(MatchUnion)' == 'true'">$(DefineConstants);MATCH_UNION</DefineConstants>
+    <DefineConstants Condition="'$(NoGenerators)' == 'true'">$(DefineConstants);NO_GENERATORS</DefineConstants>
+  </PropertyGroup>
+  <ItemGroup Condition="'$(NoGenerators)' != 'true'">
+    <PackageReference Include="CodoMetis.TypeKit" Version="2.3.1" />
+    <PackageReference Include="CodoMetis.TypeKit.Generators" Version="2.3.1" />
+  </ItemGroup>
+  <ItemGroup Condition="'$(MetalamaFramework)' != ''">
+    <PackageReference Include="Metalama.Framework" Version="$(MetalamaFramework)" />
+  </ItemGroup>
+  <ItemGroup Condition="'$(HoldUnion)' == 'true' Or '$(MatchUnion)' == 'true'">
+    <ProjectReference Include="../UnionLib/UnionLib.csproj" />
+  </ItemGroup>
+</Project>

spikes/Sdk11Rc3Metalama/App/Program.cs

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+#if PROBE_VERSION
+#error version
+#endif
+
+#if !NO_GENERATORS
+using System.Text.Json;
+using CodoMetis.TypeKit.ValueObjects;
+
+var id = OrderId.From(Guid.Parse("0b9f7d3a-5c1e-4f5a-9d8e-2a7c6b4e1f03"));
+var parsed = OrderId.Parse("0b9f7d3a-5c1e-4f5a-9d8e-2a7c6b4e1f03", null);
+var json = JsonSerializer.Serialize(id);
+var read = JsonSerializer.Deserialize<OrderId>(json);
+Console.WriteLine($"From: {id}");
+Console.WriteLine($"Parse equal: {parsed == id}");
+Console.WriteLine($"JSON: {json}, read back equal: {read == id}");
+#endif
+
+#if DECLARE_UNION
+Shape shape = new Circle(1.5);
+Console.WriteLine(shape switch
+{
+    Circle c => $"declared union: circle {c.Radius}",
+    Square s => $"declared union: square {s.Side}",
+});
+#endif
+
+#if HOLD_UNION
+var held = UnionLib.Pets.Adopt();
+Console.WriteLine($"held: {held}");
+#endif
+
+#if MATCH_UNION
+Console.WriteLine(UnionLib.Pets.Adopt() switch
+{
+    UnionLib.Cat c => $"matched union: cat {c.Name}",
+    UnionLib.Dog d => $"matched union: dog {d.Name}",
+});
+#endif
+
+#if NO_GENERATORS
+Console.WriteLine("control: no TypeKit, no Metalama");
+#endif
+
+#if !NO_GENERATORS
+public readonly partial record struct OrderId : IValue<Guid>;
+#endif
+
+#if DECLARE_UNION
+public sealed record Circle(double Radius);
+
+public sealed record Square(double Side);
+
+public union Shape(Circle, Square);
+#endif

spikes/Sdk11Rc3Metalama/Directory.Build.props

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+<Project>
+    <!-- Self-contained spike: nothing above this folder is inherited. -->
+</Project>

spikes/Sdk11Rc3Metalama/Multi/Multi.csproj

@@ -0,0 +1,32 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <!--
+      One project on both targets, with no LangVersion: each target takes its SDK default. Rows:
+        -p:MetalamaFramework=V  a direct Metalama.Framework V reference beside the 2.3.1 packages
+        -p:ProbeVersion=true    #error version, once per target
+        -p:IfUnion=true         a union and a switch over it inside #if NET11_0_OR_GREATER
+        -p:ExcludedFile=true    Net11Only/Shapes.cs, a union, compiled on net11.0 only (Compile Remove on net10.0)
+  -->
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFrameworks>net10.0;net11.0</TargetFrameworks>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <UseSharedCompilation>false</UseSharedCompilation>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DefineConstants Condition="'$(ProbeVersion)' == 'true'">$(DefineConstants);PROBE_VERSION</DefineConstants>
+    <DefineConstants Condition="'$(IfUnion)' == 'true'">$(DefineConstants);IF_UNION</DefineConstants>
+    <DefineConstants Condition="'$(ExcludedFile)' == 'true'">$(DefineConstants);EXCLUDED_FILE</DefineConstants>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="CodoMetis.TypeKit" Version="2.3.1" />
+    <PackageReference Include="CodoMetis.TypeKit.Generators" Version="2.3.1" />
+  </ItemGroup>
+  <ItemGroup Condition="'$(MetalamaFramework)' != ''">
+    <PackageReference Include="Metalama.Framework" Version="$(MetalamaFramework)" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Remove="Net11Only/**" Condition="'$(TargetFramework)' == 'net10.0'" />
+    <Compile Remove="Net11Only/**" Condition="'$(ExcludedFile)' != 'true'" />
+  </ItemGroup>
+</Project>

spikes/Sdk11Rc3Metalama/Multi/Net11Only/Shapes.cs

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+public sealed record Circle(double Radius);
+
+public sealed record Square(double Side);
+
+public union Shape(Circle, Square);
+
+public static class Shapes
+{
+    public static string Describe(Shape shape) => shape switch
+    {
+        Circle c => $"circle {c.Radius}",
+        Square s => $"square {s.Side}",
+    };
+}

spikes/Sdk11Rc3Metalama/Multi/Program.cs

@@ -0,0 +1,34 @@
+#if PROBE_VERSION
+#error version
+#endif
+
+using System.Text.Json;
+using CodoMetis.TypeKit.ValueObjects;
+
+var target = AppContext.TargetFrameworkName;
+var id = OrderId.From(Guid.Parse("0b9f7d3a-5c1e-4f5a-9d8e-2a7c6b4e1f03"));
+var json = JsonSerializer.Serialize(id);
+Console.WriteLine($"{target}: From {id}, Parse equal {OrderId.Parse(id.ToString(), null) == id}, JSON {json} read back equal {JsonSerializer.Deserialize<OrderId>(json) == id}");
+
+#if IF_UNION && NET11_0_OR_GREATER
+Pet pet = new Cat("Tom");
+Console.WriteLine(pet switch
+{
+    Cat c => $"{target}: union under #if: cat {c.Name}",
+    Dog d => $"{target}: union under #if: dog {d.Name}",
+});
+#endif
+
+#if EXCLUDED_FILE && NET11_0_OR_GREATER
+Console.WriteLine($"{target}: union in a net11.0-only file: {Shapes.Describe(new Circle(1.5))}");
+#endif
+
+public readonly partial record struct OrderId : IValue<Guid>;
+
+#if IF_UNION && NET11_0_OR_GREATER
+public sealed record Cat(string Name);
+
+public sealed record Dog(string Name);
+
+public union Pet(Cat, Dog);
+#endif