[WIP] Fixing Task.WhenEach not polyfilled on .NET Standard 2.0 when Microsoft.Bcl.AsyncInterfaces is present or .NET Standard 2.1 (#482)

* allow FeatureAsyncInterfaces to work where supported

* enable testing Task.WhenEach polyfill with FeatureAsyncInterface

Author: alastairlundy

src/Polyfill/TaskWhenEachPolyfill.cs

@@ -1,5 +1,4 @@
-#if !NET9_0_OR_GREATER && (NET5_0_OR_GREATER || NETCOREAPP3_0_OR_GREATER) && FeatureAsyncInterfaces
-
+#if !NET9_0_OR_GREATER && FeatureAsyncInterfaces
 namespace Polyfills;
 
 using System;

src/Tests/PolyfillTests_Task.cs

@@ -610,8 +610,7 @@ public async Task WaitAsync_InfiniteTimeoutAndNoCancellation_ReturnsOriginalTask
 
     #endregion
 
-#if NET5_0_OR_GREATER || NETCOREAPP3_0_OR_GREATER
-
+#if FeatureAsyncInterfaces
     #region WhenEach Tests
 
     [Test]
@@ -882,4 +881,4 @@ async Task<T> ThrowExceptionAsync<T>()
 
     #endregion
 }
-#endif
+#endif

src/Tests/Tests.csproj

@@ -11,6 +11,7 @@
     <PackageReference Include="ProjectDefaults" PrivateAssets="all" />
     <PackageReference Include="ProjectFiles" />
     <PackageReference Include="System.Memory" Condition="$(TargetFrameworkIdentifier) == '.NETStandard' or $(TargetFrameworkIdentifier) == '.NETFramework' or $(TargetFramework.StartsWith('netcoreapp'))" />
+    <PackageReference Include="Microsoft.Bcl.AsyncInterfaces" Condition="$(TargetFramework.StartsWith('net4')) OR $(TargetFramework) == 'netstandard2.0'"/>
     <PackageReference Include="System.ValueTuple" Condition="$(TargetFramework.StartsWith('net46'))" />
     <PackageReference Include="System.Net.Http" Condition="$(TargetFramework.StartsWith('net4'))" />
     <PackageReference Include="System.Threading.Tasks.Extensions" Condition="$(TargetFramework) == 'netstandard2.0' or $(TargetFramework) == 'netcoreapp2.0' or $(TargetFrameworkIdentifier) == '.NETFramework'" />