Add polyfills for C# 15 union types (UnionAttribute and IUnion)

README.md

@@ -55,6 +55,9 @@ Here's an example of some of the new features that **PolySharp** can enable down
 - `[OverloadResolutionPriority]` (needed for [overload resolution priority](https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-13#overload-resolution-priority))
 - `[ParamsCollection]` (needed for [params collection](https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-13#params-collections))
 - `[ConstantExpected]` (see [proposal](https://github.com/dotnet/runtime/issues/33771))
+- Union types (needed for [C# 15 union types](https://devblogs.microsoft.com/dotnet/csharp-15-union-types/))
+  - `[Union]`
+  - `IUnion`
 
 To leverage them, make sure to bump your C# language version. You can do this by setting the `<LangVersion>` MSBuild property in your project. For instance, by adding `<LangVersion>13.0</LangVersion>` (or your desired C# version) to the first `<PropertyGroup>` of your .csproj file. For more info on this, [see here](https://sergiopedri.medium.com/enabling-and-using-c-9-features-on-older-and-unsupported-runtimes-ce384d8debb), but remember that you don't need to manually copy polyfills anymore: simply adding a reference to **PolySharp** will do this for you automatically.
 

src/PolySharp.SourceGenerators/EmbeddedResources/System.Runtime.CompilerServices.IUnion.cs

@@ -0,0 +1,20 @@
+// <auto-generated/>
+#pragma warning disable
+#nullable enable annotations
+
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Runtime.CompilerServices
+{
+    /// <summary>
+    /// Defines the contract for union types, providing access to the underlying value.
+    /// </summary>
+    internal interface IUnion
+    {
+        /// <summary>
+        /// Gets the underlying value of the union.
+        /// </summary>
+        object? Value { get; }
+    }
+}

src/PolySharp.SourceGenerators/EmbeddedResources/System.Runtime.CompilerServices.UnionAttribute.cs

@@ -0,0 +1,21 @@
+// <auto-generated/>
+#pragma warning disable
+#nullable enable annotations
+
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Runtime.CompilerServices
+{
+    /// <summary>
+    /// Indicates that a type is a union type, allowing the C# compiler to recognize it for union type features.
+    /// </summary>
+    [global::System.AttributeUsage(
+        global::System.AttributeTargets.Class |
+        global::System.AttributeTargets.Struct,
+        AllowMultiple = false)]
+    [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+    internal sealed class UnionAttribute : global::System.Attribute
+    {
+    }
+}

tests/PolySharp.Tests/LanguageFeatures.cs

@@ -297,3 +297,22 @@ public static void AnotherCpuIntrinsic([ConstantExpected(Min = 0, Max = 8)] int
     {
     }
 }
+
+// UnionAttribute, IUnion
+[Union]
+internal sealed class TestUnion : IUnion
+{
+    private readonly object? value;
+
+    public TestUnion(int value)
+    {
+        this.value = value;
+    }
+
+    public TestUnion(string value)
+    {
+        this.value = value;
+    }
+
+    public object? Value => this.value;
+}