Check in simple end-to-end test for covariant returns.

eng/build.ps1

@@ -405,7 +405,8 @@ function TestUsingOptimizedRunner() {
   }
 
   # Exclude out the multi-targetted netcore app projects
-  $dlls = $dlls | ?{ -not ($_.FullName -match ".*netcoreapp.*") }
+  $dlls = $dlls | ?{ -not ($_.FullName -match ".*netcoreapp3.*") }
+  $dlls = $dlls | ?{ -not ($_.FullName -match ".*net5.0.*") }
 
   # Exclude out the ref assemblies
   $dlls = $dlls | ?{ -not ($_.FullName -match ".*\\ref\\.*") }

eng/targets/Imports.targets

@@ -23,7 +23,7 @@
       Only generate our runtimeconfig.json files for net core apps. It's unnecessary in desktop projects
       but gets included in lots of output items like VSIX.
     -->
-    <GenerateRuntimeConfigurationFiles Condition="'$(TargetFramework)' != 'netcoreapp3.1'">false</GenerateRuntimeConfigurationFiles>
+    <GenerateRuntimeConfigurationFiles Condition="'$(TargetFrameworkIdentifier)' != '.NETCoreApp'">false</GenerateRuntimeConfigurationFiles>
 
     <!--
       When building a .NET Core exe make sure to include the template runtimeconfig.json file 
@@ -36,7 +36,7 @@
       This condition will be evaluated multiple times in multi-targeted projects hence need to be careful
       to only set in the inner builds, not the outer build where only $(TargetFrameworks) is defined.
     -->
-    <DisableNullableWarnings Condition="'$(DisableNullableWarnings)' == '' AND $(TargetFrameworks.Contains('netcoreapp3.1')) AND '$(TargetFramework)' != '' AND '$(TargetFramework)' != 'netcoreapp3.1'">true</DisableNullableWarnings>
+    <DisableNullableWarnings Condition="'$(DisableNullableWarnings)' == '' AND '$(TargetFramework)' != '' AND '$(TargetFrameworkIdentifier)' != '.NETCoreApp'">true</DisableNullableWarnings>
 
     <!--
       Disable code style analyzers in "older" targets for a multi-targeted project. These analyzers don't

src/Compilers/CSharp/Test/Emit/Emit/CovariantReturnTests.cs

@@ -0,0 +1,310 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#if NETCOREAPP
+
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
+using Microsoft.CodeAnalysis.Test.Utilities;
+using Roslyn.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.CSharp.UnitTests.Emit
+{
+    public class CovariantReturnTests : EmitMetadataTestBase
+    {
+        private static readonly MetadataReference CorelibraryWithCovariantReturnSupport;
+
+        static CovariantReturnTests()
+        {
+            if (new CovarantReturnRuntimeOnly().ShouldSkip)
+                return;
+
+            const string corLibraryCore = @"
+namespace System
+{
+    public class Array
+    {
+        public static T[] Empty<T>() => throw null;
+    }
+    public class Console
+    {
+        public static void WriteLine(string message) => throw null;
+    }
+    public class Attribute { }
+    [Flags]
+    public enum AttributeTargets
+    {
+        Assembly = 0x1,
+        Module = 0x2,
+        Class = 0x4,
+        Struct = 0x8,
+        Enum = 0x10,
+        Constructor = 0x20,
+        Method = 0x40,
+        Property = 0x80,
+        Field = 0x100,
+        Event = 0x200,
+        Interface = 0x400,
+        Parameter = 0x800,
+        Delegate = 0x1000,
+        ReturnValue = 0x2000,
+        GenericParameter = 0x4000,
+        All = 0x7FFF
+    }
+    [AttributeUsage(AttributeTargets.Class, Inherited = true)]
+    public sealed class AttributeUsageAttribute : Attribute
+    {
+        public AttributeUsageAttribute(AttributeTargets validOn) { }
+        public bool AllowMultiple
+        {
+            get => throw null;
+            set { }
+        }
+        public bool Inherited
+        {
+            get => throw null;
+            set { }
+        }
+        public AttributeTargets ValidOn => throw null;
+    }
+    public struct Boolean { }
+    public struct Byte { }
+    public class Delegate
+    {
+        public static Delegate CreateDelegate(Type type, object firstArgument, Reflection.MethodInfo method) => null;
+    }
+    public abstract class Enum : IComparable { }
+    public class Exception
+    {
+        public Exception(string message) => throw null;
+    }
+    public class FlagsAttribute : Attribute { }
+    public delegate T Func<out T>();
+    public delegate U Func<in T, out U>(T arg);
+    public interface IComparable { }
+    public interface IDisposable
+    {
+        void Dispose();
+    }
+    public struct Int16 { }
+    public struct Int32 { }
+    public struct IntPtr { }
+    public class MulticastDelegate : Delegate { }
+    public struct Nullable<T> { }
+    public class Object
+    {
+        public virtual string ToString() => throw null;
+        public virtual int GetHashCode() => throw null;
+        public virtual bool Equals(object other) => throw null;
+    }
+    public sealed class ParamArrayAttribute : Attribute { }
+    public struct RuntimeMethodHandle { }
+    public struct RuntimeTypeHandle { }
+    public class String : IComparable { 
+        public static String Empty = null;
+        public override string ToString() => throw null;
+        public static bool operator ==(string a, string b) => throw null;
+        public static bool operator !=(string a, string b) => throw null;
+        public override bool Equals(object other) => throw null;
+        public override int GetHashCode() => throw null;
+    }
+    public class Type
+    {
+        public Reflection.FieldInfo GetField(string name) => null;
+        public static Type GetType(string name) => null;
+        public static Type GetTypeFromHandle(RuntimeTypeHandle handle) => null;
+    }
+    public class ValueType { }
+    public struct Void { }
+
+    namespace Collections
+    {
+        public interface IEnumerable
+        {
+            IEnumerator GetEnumerator();
+        }
+        public interface IEnumerator
+        {
+            object Current
+            {
+                get;
+            }
+            bool MoveNext();
+            void Reset();
+        }
+    }
+    namespace Collections.Generic
+    {
+        public interface IEnumerable<out T> : IEnumerable
+        {
+            new IEnumerator<T> GetEnumerator();
+        }
+        public interface IEnumerator<out T> : IEnumerator, IDisposable
+        {
+            new T Current
+            {
+                get;
+            }
+        }
+    }
+    namespace Linq.Expressions
+    {
+        public class Expression
+        {
+            public static ParameterExpression Parameter(Type type) => throw null;
+            public static ParameterExpression Parameter(Type type, string name) => throw null;
+            public static MethodCallExpression Call(Expression instance, Reflection.MethodInfo method, params Expression[] arguments) => throw null;
+            public static Expression<TDelegate> Lambda<TDelegate>(Expression body, params ParameterExpression[] parameters) => throw null;
+            public static MemberExpression Property(Expression expression, Reflection.MethodInfo propertyAccessor) => throw null;
+            public static ConstantExpression Constant(object value, Type type) => throw null;
+            public static UnaryExpression Convert(Expression expression, Type type) => throw null;
+        }
+        public class ParameterExpression : Expression { }
+        public class MethodCallExpression : Expression { }
+        public abstract class LambdaExpression : Expression { }
+        public class Expression<T> : LambdaExpression { }
+        public class MemberExpression : Expression { }
+        public class ConstantExpression : Expression { }
+        public sealed class UnaryExpression : Expression { }
+    }
+    namespace Reflection
+    {
+        public class AssemblyVersionAttribute : Attribute
+        {
+            public AssemblyVersionAttribute(string version) { }
+        }
+        public class DefaultMemberAttribute : Attribute
+        {
+            public DefaultMemberAttribute(string name) { }
+        }
+        public abstract class MemberInfo { }
+        public abstract class MethodBase : MemberInfo
+        {
+            public static MethodBase GetMethodFromHandle(RuntimeMethodHandle handle) => throw null;
+        }
+        public abstract class MethodInfo : MethodBase
+        {
+            public virtual Delegate CreateDelegate(Type delegateType, object target) => throw null;
+        }
+        public abstract class FieldInfo : MemberInfo
+        {
+            public abstract object GetValue(object obj);
+        }
+    }
+    namespace Runtime.CompilerServices
+    {
+        public static class RuntimeHelpers
+        {
+            public static object GetObjectValue(object obj) => null;
+        }
+    }
+}
+";
+            const string corlibWithCovariantSupport = corLibraryCore + @"
+namespace System.Runtime.CompilerServices
+{
+    public static class RuntimeFeature
+    {
+        public const string CovariantReturnsOfClasses = nameof(CovariantReturnsOfClasses);
+        public const string DefaultImplementationsOfInterfaces = nameof(DefaultImplementationsOfInterfaces);
+    }
+    public sealed class PreserveBaseOverridesAttribute : Attribute { }
+}
+";
+            var compilation = CreateEmptyCompilation(new string[] {
+                corlibWithCovariantSupport,
+                @"[assembly: System.Reflection.AssemblyVersion(""4.0.0.0"")]"
+            }, assemblyName: "mscorlib");
+            compilation.VerifyDiagnostics();
+            CorelibraryWithCovariantReturnSupport = compilation.EmitToImageReference(options: new CodeAnalysis.Emit.EmitOptions(runtimeMetadataVersion: "v5.1"));
+        }
+
+        private static CSharpCompilation CreateCovariantCompilation(
+            string source,
+            CSharpCompilationOptions options = null,
+            IEnumerable<MetadataReference> references = null)
+        {
+            Assert.NotNull(CorelibraryWithCovariantReturnSupport);
+            references = (references == null) ?
+                new[] { CorelibraryWithCovariantReturnSupport } :
+                references.ToArray().Prepend(CorelibraryWithCovariantReturnSupport);
+            return CreateEmptyCompilation(
+                source,
+                options: options,
+                parseOptions: TestOptions.WithCovariantReturns,
+                references: references);
+        }
+
+        [ConditionalFact(typeof(CovarantReturnRuntimeOnly))]
+        public void SimpleCovariantReturnEndToEndTest()
+        {
+            var source = @"
+using System;
+class Base
+{
+    public virtual object M() => ""Base.M"";
+}
+class Derived : Base
+{
+    public override string M() => ""Derived.M"";
+}
+class Program
+{
+    static void Main()
+    {
+        Derived d = new Derived();
+        Base b = d;
+        string s = d.M();
+        object o = b.M();
+        Console.WriteLine(s.ToString());
+        Console.WriteLine(o.ToString());
+    }
+}
+";
+            var compilation = CreateCovariantCompilation(source, options: TestOptions.DebugExe);
+            compilation.VerifyDiagnostics();
+            var expectedOutput =
+@"Derived.M
+Derived.M";
+            CompileAndVerify(compilation, expectedOutput: expectedOutput, verify: Verification.Skipped);
+        }
+
+        [ConditionalFact(typeof(CovarantReturnRuntimeOnly))]
+        public void CovariantRuntimeHasRequiredMembers()
+        {
+            var source = @"
+using System;
+class Base
+{
+    public virtual object M() => ""Base.M"";
+}
+class Derived : Base
+{
+    public override string M() => ""Derived.M"";
+}
+class Program
+{
+    static void Main()
+    {
+        var value = (string)Type.GetType(""System.Runtime.CompilerServices.RuntimeFeature"").GetField(""CovariantReturnsOfClasses"").GetValue(null);
+        if (value != ""CovariantReturnsOfClasses"")
+            throw new Exception(value.ToString());
+
+        var attr = Type.GetType(""System.Runtime.CompilerServices.PreserveBaseOverridesAttribute"");
+        if (attr == null)
+            throw new Exception(""missing System.Runtime.CompilerServices.PreserveBaseOverridesAttribute"");
+    }
+}
+";
+            var compilation = CreateCovariantCompilation(source, options: TestOptions.DebugExe);
+            compilation.VerifyDiagnostics();
+            var expectedOutput = @"";
+            CompileAndVerify(compilation, expectedOutput: expectedOutput, verify: Verification.Skipped);
+        }
+    }
+}
+
+#endif

src/Compilers/CSharp/Test/Emit/Microsoft.CodeAnalysis.CSharp.Emit.UnitTests.csproj

@@ -4,7 +4,7 @@
   <PropertyGroup>
     <OutputType>Library</OutputType>
     <RootNamespace>Microsoft.CodeAnalysis.CSharp.UnitTests</RootNamespace>
-    <TargetFrameworks>netcoreapp3.1;net472</TargetFrameworks>
+    <TargetFrameworks>net5.0;net472</TargetFrameworks>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
   <ItemGroup Label="Project References">

src/Test/Utilities/Portable/Assert/ConditionalFactAttribute.cs

@@ -260,6 +260,19 @@ public class WindowsDesktopOnly : ExecutionCondition
         public override string SkipReason => "Test only supported on Windows desktop";
     }
 
+    public class CovarantReturnRuntimeOnly : ExecutionCondition
+    {
+        public override bool ShouldSkip
+        {
+            get
+            {
+                // See if the runtime supports covariant returns.
+                return Type.GetType("System.Runtime.CompilerServices.RuntimeFeature")?.GetField("CovariantReturnsOfClasses") == null;
+            }
+        }
+        public override string SkipReason => "Test only supported on runtimes that support covariant returns";
+    }
+
     public class UnixLikeOnly : ExecutionCondition
     {
         public override bool ShouldSkip => !PathUtilities.IsUnixLikePlatform;

src/Tools/BuildBoss/ProjectCheckerUtil.cs

@@ -274,6 +274,7 @@ private bool CheckTargetFrameworks(TextWriter textWriter)
                     case "net20":
                     case "net472":
                     case "netcoreapp3.1":
+                    case "net5.0":
                         continue;
                 }