Fix type forwarding

Author: stephentoub

src/libraries/System.Collections/ref/System.Collections.Forwards.cs

@@ -0,0 +1,7 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// ------------------------------------------------------------------------------
+// Changes to this file must follow the https://aka.ms/api-review process.
+// ------------------------------------------------------------------------------
+
+[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(System.Collections.ObjectModel.ReadOnlySet<>))]

src/libraries/System.Collections/ref/System.Collections.csproj

@@ -4,6 +4,7 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="System.Collections.cs" />
+    <Compile Include="System.Collections.Forwards.cs" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\System.Runtime\ref\System.Runtime.csproj" />

src/libraries/System.Collections/tests/Generic/CollectionExtensionsTests.cs

@@ -122,6 +122,8 @@ public void AsReadOnly_TurnsISetIntoReadOnlySet()
             ISet<string> set = new HashSet<string> { "A", "B" };
             ReadOnlySet<string> readOnlySet = set.AsReadOnly();
             Assert.NotNull(readOnlySet);
+            Assert.NotSame(set, readOnlySet);
+            Assert.NotSame(readOnlySet, set.AsReadOnly());
             CollectionAsserts.Equal(set, readOnlySet);
         }