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

README.md

@@ -59,6 +59,9 @@ Here's an example of some of the new features that **PolySharp** can enable down
 - `[ExtensionMarker]` (needed for [extensions](https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-14#extension-members))
 - `[IsClosedType]` (needed for [closed hierarchies](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-15.0/closed-hierarchies))
 - `[RequiresUnsafe]` (needed for [unsafe evolution](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/unsafe-evolution))
+- Union types (needed for [union types](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/union))
+  - `[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>15.0</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.Package/README.md

@@ -57,6 +57,9 @@ Here's an example of some of the new features that **PolySharp** can enable down
 - `[ExtensionMarker]` (needed for [extensions](https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-14#extension-members))
 - `[IsClosedType]` (needed for [closed hierarchies](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-15.0/closed-hierarchies))
 - `[RequiresUnsafe]` (needed for [unsafe evolution](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/unsafe-evolution))
+- Union types (needed for [union types](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/union))
+  - `[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>15.0</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/LanguageSupport/System.Runtime.CompilerServices.IUnion.cs

@@ -0,0 +1,31 @@
+// <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>
+    /// Provides a common interface for accessing the contents of a union type at runtime.
+    /// </summary>
+    /// <remarks>
+    /// The C# compiler automatically implements this interface on types generated by union declarations.
+    /// User-defined types annotated with <see cref="global::System.Runtime.CompilerServices.UnionAttribute"/> may also implement this interface
+    /// to provide runtime access to the union's value. Implementing this interface is not required
+    /// for union behaviors provided by the compiler.
+    /// </remarks>
+    /// <seealso cref="global::System.Runtime.CompilerServices.UnionAttribute"/>
+    [global::Microsoft.CodeAnalysis.Embedded]
+    internal interface IUnion
+    {
+        /// <summary>
+        /// Gets the value contained in the union, or <see langword="null"/> if the union has no value.
+        /// </summary>
+        /// <value>
+        /// The current value of the union as one of its case types, or <see langword="null"/>.
+        /// </value>
+        object? Value { get; }
+    }
+}

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

@@ -0,0 +1,29 @@
+// <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 class or struct is a union type, enabling compiler support for union behaviors.
+    /// </summary>
+    /// <remarks>
+    /// Any class or struct annotated with this attribute is recognized by the C# compiler as a union type.
+    /// Union types may support behaviors such as implicit conversions from case types, pattern matching
+    /// that unwraps the union's contents, and switch exhaustiveness checking.
+    /// </remarks>
+    /// <seealso cref="global::System.Runtime.CompilerServices.IUnion"/>
+    [global::System.AttributeUsage(
+        global::System.AttributeTargets.Class |
+        global::System.AttributeTargets.Struct,
+        AllowMultiple = false,
+        Inherited = false)]
+    [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+    [global::Microsoft.CodeAnalysis.Embedded]
+    internal sealed class UnionAttribute : global::System.Attribute
+    {
+    }
+}

tests/PolySharp.Tests/LanguageFeatures.cs

@@ -305,6 +305,99 @@ public static void AnotherCpuIntrinsic([ConstantExpected(Min = 0, Max = 8)] int
     }
 }
 
+// UnionAttribute, IUnion
+internal static class UnionTests
+{
+    public static int Test()
+    {
+        IntOrString number = 42;
+        IntOrString text = "PolySharp";
+        IUnion union = number;
+
+        _ = union.Value;
+        _ = GetValue(number);
+        _ = GetValue(default);
+
+        return GetValue(text);
+    }
+
+    public static int GetValue(IntOrString value)
+    {
+        return value switch
+        {
+            int number => number,
+            string text => text.Length,
+            null => 0
+        };
+    }
+
+    public static int TestManualUnion()
+    {
+        IntOrBool number = 42;
+        IntOrBool boolean = true;
+
+        _ = GetManualValue(number);
+        _ = GetManualValue(default);
+
+        return GetManualValue(boolean);
+    }
+
+    public static int GetManualValue(IntOrBool value)
+    {
+        return value switch
+        {
+            int number => number,
+            bool boolean => boolean ? 1 : 0,
+            null => -1
+        };
+    }
+
+    public readonly union IntOrString(int, string);
+
+    [Union]
+    public readonly struct IntOrBool : IUnion
+    {
+        private readonly int number;
+        private readonly bool boolean;
+        private readonly byte tag;
+
+        public IntOrBool(int value)
+        {
+            this.number = value;
+            this.tag = 1;
+        }
+
+        public IntOrBool(bool value)
+        {
+            this.boolean = value;
+            this.tag = 2;
+        }
+
+        public object? Value => this.tag switch
+        {
+            1 => this.number,
+            2 => this.boolean,
+            _ => null
+        };
+
+        public bool HasValue => this.tag != 0;
+
+        public bool TryGetValue(out int value)
+        {
+            value = this.number;
+
+            return this.tag == 1;
+        }
+
+        public bool TryGetValue(out bool value)
+        {
+            value = this.boolean;
+
+            return this.tag == 2;
+        }
+    }
+}
+
 [CompilerLoweringPreserve]
 internal sealed class TestFlowingAttribute : Attribute;