Add new System.Diagnostics.UnreachableException polyfill

The System.Diagnostics.UnreachableException.cs file was not actually auto-generated (see Sergio0694#88).

Fixes Sergio0694#60

README.md

@@ -55,6 +55,7 @@ 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))
+- `UnreachableException` (see [docs](https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.unreachableexception))
 
 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.Package/README.md

@@ -53,6 +53,7 @@ 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))
+- `UnreachableException` (see [docs](https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.unreachableexception))
 
 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.Diagnostics.UnreachableException.cs

@@ -0,0 +1,43 @@
+// <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.Diagnostics
+{
+    /// <summary>
+    /// Exception thrown when the program executes an instruction that was thought to be unreachable.
+    /// </summary>
+    [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+    internal sealed class UnreachableException : Exception
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="UnreachableException"/> class with the default error message.
+        /// </summary>
+        public UnreachableException()
+            : base("The program executed an instruction that was thought to be unreachable.")
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="UnreachableException"/> class with a specified error message.
+        /// </summary>
+        /// <param name="message">The error message that explains the reason for the exception.</param>
+        public UnreachableException(string? message)
+            : base(message)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="UnreachableException"/> class with a specified error message and a reference to the inner exception that is the cause of this exception.
+        /// </summary>
+        /// <param name="message">The error message that explains the reason for the exception.</param>
+        /// <param name="innerException">The exception that is the cause of the current exception.</param>
+        public UnreachableException(string? message, Exception? innerException)
+            : base(message, innerException)
+        {
+        }
+    }
+}

tests/PolySharp.Tests/LanguageFeatures.cs

@@ -1,8 +1,10 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
+using System.Runtime.ExceptionServices;
 using System.Runtime.Versioning;
 #if !NETSTANDARD2_1
 using System.Threading.Tasks;
@@ -38,6 +40,13 @@ public void Throws([DoesNotReturnIf(true)] bool value)
     {
     }
 
+    // UnreachableException
+    public object Unreachable()
+    {
+        ExceptionDispatchInfo.Capture(new Exception()).Throw();
+        throw new UnreachableException();
+    }
+
     // MaybeNullAttribute
     [return: MaybeNull]
     public string ModifyValue()