Add a reference assembly to hide the methods we're carrying internally in the 2.1/3.0 lib

Ix.NET/Source/Ix.NET.sln

@@ -1,7 +1,7 @@
 
 Microsoft Visual Studio Solution File, Format Version 12.00
-# Visual Studio 15
-VisualStudioVersion = 15.0.26730.3
+# Visual Studio Version 16
+VisualStudioVersion = 16.0.28606.126
 MinimumVisualStudioVersion = 10.0.40219.1
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "test", "test", "{87534290-A7A6-47A4-9A3A-D0D21A9AD1D4}"
 EndProject
@@ -60,6 +60,8 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.Interactive.Provider
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Benchmarks.System.Interactive", "Benchmarks.System.Interactive\Benchmarks.System.Interactive.csproj", "{3285529A-8227-4D40-B524-1A1F919F0E7B}"
 EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.Linq.Async.Ref", "refs\System.Linq.Async.Ref\System.Linq.Async.Ref.csproj", "{1754B36C-D0DB-4E5D-8C30-1F116046DC0F}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -130,6 +132,10 @@ Global
 		{3285529A-8227-4D40-B524-1A1F919F0E7B}.Debug|Any CPU.Build.0 = Current Sources|Any CPU
 		{3285529A-8227-4D40-B524-1A1F919F0E7B}.Release|Any CPU.ActiveCfg = Current Sources|Any CPU
 		{3285529A-8227-4D40-B524-1A1F919F0E7B}.Release|Any CPU.Build.0 = Current Sources|Any CPU
+		{1754B36C-D0DB-4E5D-8C30-1F116046DC0F}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{1754B36C-D0DB-4E5D-8C30-1F116046DC0F}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{1754B36C-D0DB-4E5D-8C30-1F116046DC0F}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{1754B36C-D0DB-4E5D-8C30-1F116046DC0F}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -151,6 +157,7 @@ Global
 		{1C966C34-0AA1-44D8-9E7D-C7866958F2D6} = {61683064-A120-44A7-A174-E19154F6D84F}
 		{2EC0C302-B029-4DDB-AC91-000BF11006AD} = {A3D72E6E-4ADA-42E0-8B2A-055B1F244281}
 		{5DF341BE-B369-4250-AFD4-604DE8C95E45} = {A3D72E6E-4ADA-42E0-8B2A-055B1F244281}
+		{1754B36C-D0DB-4E5D-8C30-1F116046DC0F} = {A3D72E6E-4ADA-42E0-8B2A-055B1F244281}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {AF70B0C6-C9D9-43B1-9BE4-08720EC1B7B7}

Ix.NET/Source/System.Interactive.Async/System.Interactive.Async.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.SDK">
 
   <PropertyGroup>
     <Description>Interactive Extensions Async Library used to express queries over asynchronous enumerable sequences.</Description>

Ix.NET/Source/System.Linq.Async/System.Linq.Async.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="MSBuild.Sdk.Extras">
 
   <PropertyGroup>
     <TargetFrameworks>net45;net46;netstandard1.4;netstandard2.0;netcoreapp3.0</TargetFrameworks>
@@ -19,6 +19,7 @@
 
   <ItemGroup>
     <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.2" />
+    <ReferenceAssemblyProjectReference Include="..\refs\System.Linq.Async.Ref\System.Linq.Async.Ref.csproj" />
   </ItemGroup>
 
   <ItemGroup>

Ix.NET/Source/System.Linq.Async/System/Threading/Tasks/AsyncEnumerableExtensions.cs

@@ -48,11 +48,35 @@ namespace System.Threading.Tasks
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static ConfiguredCancelableAsyncEnumerable<T> WithCancellation<T>(
             this IAsyncEnumerable<T> source, CancellationToken cancellationToken) => TaskExtensions.WithCancellation(source, cancellationToken);
+
+#else
+        // Reference assembly, these won't be emmited, but keep these internal so we can compile
+        /// <summary>Configures how awaits on the tasks returned from an async iteration will be performed.</summary>
+        /// <typeparam name="T">The type of the objects being iterated.</typeparam>
+        /// <param name="source">The source enumerable being iterated.</param>
+        /// <param name="continueOnCapturedContext">Whether to capture and marshal back to the current context.</param>
+        /// <returns>The configured enumerable.</returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static ConfiguredCancelableAsyncEnumerable<T> ConfigureAwait<T>(
+            this IAsyncEnumerable<T> source, bool continueOnCapturedContext) => TaskExtensions.ConfigureAwait(source, continueOnCapturedContext);
+
+        /// <summary>Sets the <see cref="CancellationToken"/> to be passed to <see cref="IAsyncEnumerable{T}.GetAsyncEnumerator(CancellationToken)"/> when iterating.</summary>
+        /// <typeparam name="T">The type of the objects being iterated.</typeparam>
+        /// <param name="source">The source enumerable being iterated.</param>
+        /// <param name="cancellationToken">The <see cref="CancellationToken"/> to use.</param>
+        /// <returns>The configured enumerable.</returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static ConfiguredCancelableAsyncEnumerable<T> WithCancellation<T>(
+            this IAsyncEnumerable<T> source, CancellationToken cancellationToken) => TaskExtensions.WithCancellation(source, cancellationToken);
 #endif
 
         public static ConfiguredCancelableAsyncEnumerable<T>.Enumerator GetConfiguredAsyncEnumerator<T>(this IAsyncEnumerable<T> enumerable, CancellationToken cancellationToken, bool continueOnCapturedContext)
         {
+#if REFERENCE_ASSEMBLY
+            return default;
+#else
             return ConfigureAwait(enumerable, continueOnCapturedContext).WithCancellation(cancellationToken).GetAsyncEnumerator();
+#endif
         }
     }
 }

Ix.NET/Source/refs/System.Linq.Async.Ref/System.Linq.Async.Ref.csproj

@@ -4,12 +4,12 @@
     <Description>LINQ Standard Query Operators used to express queries over asynchronous enumerable sequences.</Description>
     <AssemblyTitle>System.Linq.Async</AssemblyTitle>
     <Authors>Microsoft</Authors>
-    <TargetFrameworks>netstandard1.0;netcoreapp2.0;netstandard2.1</TargetFrameworks>
+    <TargetFrameworks>net45;netstandard1.4;netcoreapp3.0</TargetFrameworks>
     <PackageTags>Enumerable;Asynchronous;LINQ</PackageTags>
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.1" />
+    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.2" />
   </ItemGroup>
 
   <ItemGroup>