Ensure IsExternalInit is type forwarded on NET builds (#619)

It's included as internal on netstandard/net472, and the C# compiler may bake a reference to that into a consumer. If that consumer is then used with a net8.0+ build, the IsExternalInit needs to be there and forwarded to the real one.

This switches our polyfill files to always be included but the actual contents ifdef'd out on TFMs that already have the contents. That then makes it easier to do specialized ifdef'ing in the future, as this does for IsExternalInit.

src/Common/Polyfills/System/Collections/Generic/CollectionExtensions.cs

@@ -1,3 +1,4 @@
+#if !NET
 using ModelContextProtocol;
 
 namespace System.Collections.Generic;
@@ -18,4 +19,5 @@ internal static class CollectionExtensions
 
     public static Dictionary<TKey, TValue> ToDictionary<TKey, TValue>(this IEnumerable<KeyValuePair<TKey, TValue>> source) =>
         source.ToDictionary(kv => kv.Key, kv => kv.Value);
-}
+}
+#endif

src/Common/Polyfills/System/Diagnostics/CodeAnalysis/DynamicallyAccessedMemberTypes.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Diagnostics.CodeAnalysis;
 
 /// <summary>
@@ -162,3 +163,4 @@ internal enum DynamicallyAccessedMemberTypes
     /// </summary>
     All = ~None
 }
+#endif

src/Common/Polyfills/System/Diagnostics/CodeAnalysis/DynamicallyAccessedMembersAttribute.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Diagnostics.CodeAnalysis;
 
 /// <summary>
@@ -48,3 +49,4 @@ internal sealed class DynamicallyAccessedMembersAttribute : Attribute
     /// </summary>
     public DynamicallyAccessedMemberTypes MemberTypes { get; }
 }
+#endif

src/Common/Polyfills/System/Diagnostics/CodeAnalysis/NullableAttributes.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Diagnostics.CodeAnalysis
 {
     /// <summary>Specifies that null is allowed as an input even if the corresponding type disallows it.</summary>
@@ -137,3 +138,4 @@ namespace System.Diagnostics.CodeAnalysis
         public string[] Members { get; }
     }
 }
+#endif

src/Common/Polyfills/System/Diagnostics/CodeAnalysis/RequiresDynamicCodeAttribute.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Diagnostics.CodeAnalysis;
 
 /// <summary>
@@ -36,3 +37,4 @@ internal sealed class RequiresDynamicCodeAttribute : Attribute
     /// </summary>
     public string? Url { get; set; }
 }
+#endif

src/Common/Polyfills/System/Diagnostics/CodeAnalysis/RequiresUnreferencedCode.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Diagnostics.CodeAnalysis;
 
 /// <summary>
@@ -37,3 +38,4 @@ internal sealed class RequiresUnreferencedCodeAttribute : Attribute
     /// </summary>
     public string? Url { get; set; }
 }
+#endif

src/Common/Polyfills/System/Diagnostics/CodeAnalysis/SetsRequiredMembersAttribute.cs

@@ -1,5 +1,7 @@
+#if !NET
 namespace System.Diagnostics.CodeAnalysis
 {
     [AttributeUsage(AttributeTargets.Constructor, AllowMultiple = false, Inherited = false)]
     internal sealed class SetsRequiredMembersAttribute : Attribute;
 }
+#endif

src/Common/Polyfills/System/Diagnostics/CodeAnalysis/StringSyntaxAttribute.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Diagnostics.CodeAnalysis;
 
 /// <summary>Specifies the syntax used in a string.</summary>
@@ -65,4 +66,5 @@ internal sealed class StringSyntaxAttribute : Attribute
 
     /// <summary>The syntax identifier for strings containing XML.</summary>
     public const string Xml = nameof(Xml);
-}
+}
+#endif

src/Common/Polyfills/System/Diagnostics/CodeAnalysis/UnconditionalSuppressMessageAttribute.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Diagnostics.CodeAnalysis;
 
 /// <summary>
@@ -81,4 +82,5 @@ internal sealed class UnconditionalSuppressMessageAttribute : Attribute
     /// Gets or sets the justification for suppressing the code analysis message.
     /// </summary>
     public string? Justification { get; set; }
-}
+}
+#endif

src/Common/Polyfills/System/IO/StreamExtensions.cs

@@ -3,6 +3,7 @@ using System.Buffers;
 using System.Runtime.InteropServices;
 using System.Text;
 
+#if !NET
 namespace System.IO;
 
 internal static class StreamExtensions
@@ -61,4 +62,5 @@ internal static class StreamExtensions
             }
         }
     }
-}
+}
+#endif

src/Common/Polyfills/System/IO/TextWriterExtensions.cs

@@ -1,3 +1,4 @@
+#if !NET
 namespace System.IO;
 
 internal static class TextWriterExtensions
@@ -7,4 +8,5 @@ internal static class TextWriterExtensions
         cancellationToken.ThrowIfCancellationRequested();
         await writer.FlushAsync();
     }
-}
+}
+#endif

src/Common/Polyfills/System/Net/Http/HttpClientExtensions.cs

@@ -1,3 +1,4 @@
+#if !NET
 using ModelContextProtocol;
 
 namespace System.Net.Http;
@@ -19,4 +20,5 @@ internal static class HttpClientExtensions
         cancellationToken.ThrowIfCancellationRequested();
         return await content.ReadAsStringAsync();
     }
-}
+}
+#endif

src/Common/Polyfills/System/PasteArguments.cs

@@ -5,6 +5,7 @@
 // https://github.com/dotnet/runtime/blob/d2650b6ae7023a2d9d2c74c56116f1f18472ab04/src/libraries/System.Private.CoreLib/src/System/PasteArguments.cs
 // and changed from using ValueStringBuilder to StringBuilder.
 
+#if !NET
 using System.Text;
 
 namespace System;
@@ -98,4 +99,5 @@ internal static partial class PasteArguments
 
     private const char Quote = '\"';
     private const char Backslash = '\\';
-}
+}
+#endif

src/Common/Polyfills/System/Runtime/CompilerServices/CallerArgumentExpressionAttribute.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Runtime.CompilerServices;
 
 [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = false)]
@@ -13,3 +14,4 @@ internal sealed class CallerArgumentExpressionAttribute : Attribute
 
     public string ParameterName { get; }
 }
+#endif

src/Common/Polyfills/System/Runtime/CompilerServices/CompilerFeatureRequiredAttribute.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 namespace System.Runtime.CompilerServices
 {
     /// <summary>
@@ -30,3 +31,4 @@ namespace System.Runtime.CompilerServices
         public const string RequiredMembers = nameof(RequiredMembers);
     }
 }
+#endif

src/Common/Polyfills/System/Runtime/CompilerServices/DefaultInterpolatedStringHandler.cs

@@ -5,6 +5,7 @@
 // https://github.com/dotnet/runtime/blob/dd75c45c123055baacd7aa4418f425f412797a29/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/DefaultInterpolatedStringHandler.cs
 // and then modified to build on netstandard2.0.
 
+#if !NET
 using System.Buffers;
 using System.Diagnostics;
 using System.Globalization;
@@ -614,4 +615,5 @@ namespace System.Runtime.CompilerServices
             return value;
         }
     }
-}
+}
+#endif

src/Common/Polyfills/System/Runtime/CompilerServices/IsExternalInit.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 using System.ComponentModel;
 
 namespace System.Runtime.CompilerServices
@@ -12,3 +13,8 @@ namespace System.Runtime.CompilerServices
     [EditorBrowsable(EditorBrowsableState.Never)]
     internal static class IsExternalInit;
 }
+#else
+// The compiler emits a reference to the internal copy of this type in the non-.NET builds,
+// so we must include a forward to be compatible.
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Runtime.CompilerServices.IsExternalInit))]
+#endif

src/Common/Polyfills/System/Runtime/CompilerServices/RequiredMemberAttribute.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#if !NET
 using System.ComponentModel;
 
 namespace System.Runtime.CompilerServices
@@ -10,3 +11,4 @@ namespace System.Runtime.CompilerServices
     [EditorBrowsable(EditorBrowsableState.Never)]
     internal sealed class RequiredMemberAttribute : Attribute;
 }
+#endif

src/Common/Polyfills/System/Threading/CancellationTokenSourceExtensions.cs

@@ -1,3 +1,4 @@
+#if !NET
 using ModelContextProtocol;
 
 namespace System.Threading.Tasks;
@@ -11,4 +12,5 @@ internal static class CancellationTokenSourceExtensions
         cancellationTokenSource.Cancel();
         return Task.CompletedTask;
     }
-}
+}
+#endif

src/Common/Polyfills/System/Threading/Channels/ChannelExtensions.cs

@@ -1,3 +1,4 @@
+#if !NET
 using System.Runtime.CompilerServices;
 
 namespace System.Threading.Channels;
@@ -14,4 +15,5 @@ internal static class ChannelExtensions
             }
         }
     }
-}
+}
+#endif