Don't set "platform library" for .NET Core 3.0 and higher

src/Tasks/Microsoft.NET.Build.Tasks/ResolvePackageAssets.cs

@@ -1194,19 +1194,6 @@ private HashSet<string> GetPlatformPackageExclusions()
                     }
                 }
 
-                // Exclude any runtime frameworks
-                if (_task.RuntimeFrameworks != null)
-                {
-                    foreach (var framework in _task.RuntimeFrameworks)
-                    {
-                        var frameworkLibrary = _runtimeTarget.GetLibrary(framework.ItemSpec);
-                        if (frameworkLibrary != null)
-                        {
-                            packageExclusions.UnionWith(_runtimeTarget.GetPlatformExclusionList(frameworkLibrary, libraryLookup));
-                        }
-                    }
-                }
-
                 return packageExclusions;
             }
 

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

@@ -661,14 +661,16 @@ Copyright (c) .NET Foundation. All rights reserved.
                            _DefaultMicrosoftNETPlatformLibrary
 
   .NET Core apps can have shared frameworks that are pre-installed on the target machine, thus the app is "portable"
-  to any machine that already has the shared framework installed. In order to enable this, a "platform" library
-  has to be declared. The platform library and its dependencies will be excluded from the runtime assemblies.
+  to any machine that already has the shared framework installed. For .NET Core 1.x and 2.x, a "platform" library
+  is declared. The platform library and its dependencies will be excluded from the publish output.
+
+  For .NET Core 3 and up, targeting packs and runtime packs are used for shared framework assets instead of PackageReference
   ============================================================
   -->
   <Target Name="_DefaultMicrosoftNETPlatformLibrary">
 
     <PropertyGroup Condition="'$(MicrosoftNETPlatformLibrary)' == ''">
-      <MicrosoftNETPlatformLibrary Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp'">Microsoft.NETCore.App</MicrosoftNETPlatformLibrary>
+      <MicrosoftNETPlatformLibrary Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp' And '$(_TargetFrameworkVersionWithoutV)' &lt; '3.0'">Microsoft.NETCore.App</MicrosoftNETPlatformLibrary>
     </PropertyGroup>
 
   </Target>

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

@@ -638,6 +638,41 @@ public void It_uses_lowercase_form_of_the_target_framework_for_the_output_path()
                 .BeEquivalentTo("netcoreapp1.1");
         }
 
+        [Fact]
+        public void BuildWithTransitiveReferenceToNetCoreAppPackage()
+        {
+            var testProject = new TestProject()
+            {
+                Name = "NetCoreAppPackageReference",
+                TargetFrameworks = "netcoreapp3.0",
+                IsSdkProject = true,
+                IsExe = true
+            };
+
+            var referencedProject = new TestProject()
+            {
+                Name = "NetStandardProject",
+                TargetFrameworks = "netstandard2.0",
+                IsSdkProject = true,
+                IsExe = false
+            };
+
+            //  The SharpDX package depends on the Microsoft.NETCore.App package
+            referencedProject.PackageReferences.Add(new TestPackageReference("SharpDX", "4.0.1"));
+
+            testProject.ReferencedProjects.Add(referencedProject);
+
+            var testAsset = _testAssetsManager.CreateTestProject(testProject, testProject.Name)
+                .Restore(Log, testProject.Name);
+
+            var buildCommand = new BuildCommand(Log, Path.Combine(testAsset.TestRoot, testProject.Name));
+
+            buildCommand
+                .Execute()
+                .Should()
+                .Pass();
+        }
+
         private static bool IsPE32(string path)
         {
             using (var reader = new PEReader(File.OpenRead(path)))