Default ProduceReferenceAssembly to true

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.TargetFrameworkInference.targets

@@ -211,6 +211,10 @@ Copyright (c) .NET Foundation. All rights reserved.
     <AppendTargetFrameworkToOutputPath Condition="'$(AppendTargetFrameworkToOutputPath)' == ''">true</AppendTargetFrameworkToOutputPath>
   </PropertyGroup>
 
+  <PropertyGroup>
+    <ProduceReferenceAssembly Condition="'$(ProduceReferenceAssembly)' == '' and '$(TargetFrameworkIdentifier)' == '.NETCoreApp' and $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), 5.0))" >true</ProduceReferenceAssembly>
+  </PropertyGroup>
+
   <!--
     Append $(TargetFramework) directory to output and intermediate paths to prevent bin clashes between
     targets.

src/Tests/Microsoft.NET.Build.Tests/GivenThatWeWantToProduceReferenceAssembly.cs

@@ -0,0 +1,44 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.IO;
+using FluentAssertions;
+using Microsoft.NET.TestFramework;
+using Microsoft.NET.TestFramework.Commands;
+using Xunit;
+using Xunit.Abstractions;
+using Microsoft.NET.TestFramework.Assertions;
+using Microsoft.NET.TestFramework.ProjectConstruction;
+using System.Collections.Generic;
+
+namespace Microsoft.NET.Build.Tests
+{
+    public class GivenThatWeWantToProduceReferenceAssembly : SdkTest
+    {
+        public GivenThatWeWantToProduceReferenceAssembly(ITestOutputHelper log) : base(log)
+        {}
+
+        [Theory]
+        [InlineData("netcoreapp3.1", false)]
+        [InlineData("net5.0", true)]
+        public void It_produces_ref_assembly_for_appropriate_frameworks(string targetFramework, bool expectedExists)
+        {
+            TestProject testProject = new TestProject()
+            {
+                Name = "ProduceRefAssembly",
+                IsSdkProject = true, 
+                IsExe = true, 
+                TargetFrameworks = targetFramework
+            };
+
+            var testAsset = _testAssetsManager.CreateTestProject(testProject);
+
+            var buildCommand = new BuildCommand(testAsset);
+            buildCommand.Execute()
+                .Should()
+                .Pass();
+            var filePath = Path.Combine(testAsset.Path, testProject.Name, "obj", "Debug", targetFramework, "ref", $"{testProject.Name}.dll");
+            File.Exists(filePath).Should().Be(expectedExists);
+        }
+    }
+}