Make all innner build RIDs visible to restore

Fix #871

Author: nguerrera

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

@@ -352,6 +352,14 @@ Copyright (c) .NET Foundation. All rights reserved.
 
   </Target>
 
+  <!--
+  ============================================================
+                                      GetAllRuntimeIdentifiers
+  ============================================================
+  -->
+  <Target Name="GetAllRuntimeIdentifiers" 
+          Returns="$(RuntimeIdentifiers);$(RuntimeIdentifier)" />
+
   <!--
   ============================================================
                                          Project Capabilities

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

@@ -30,5 +30,47 @@ Copyright (c) .NET Foundation. All rights reserved.
   <Target Name="Publish">
     <Error Text="The 'Publish' target is not supported without specifying a target framework. The current project targets multiple frameworks, please specify the framework for the published application." />
   </Target>
+
+  ============================================================
+                                      GetAllRuntimeIdentifiers
+
+  Outer build implementation of GetAllRuntimeIdentifiers returns
+  a union of all runtime identifiers used across inner and outer
+  build evaluations.
+
+  It is further set to run before '_GenerateRestoreProjecSpec' 
+  (note that running only 'Restore' is too late and will not work
+  with solution level restore). This ensures that any conditioning
+  of runtime  identifiers against TargetFramework does not prevent
+  restore from providing  the necessary RID-specific assets for all
+  inner builds.
 
+  It also brings parity to VS vs. command line behavior in this
+  scenario because VS passes all of the information from each
+  configured inner build to restore, whereas command-line restore
+  without this target would only use the runtime identifiers that
+  are statically set in the outer evaluation.
+  ============================================================
+
+  <Target Name="GetAllRuntimeIdentifiers" 
+          Returns="$(RuntimeIdentifiers)"
+          BeforeTargets="_GenerateRestoreProjectSpec">
+
+    <ItemGroup>
+      <_GetAllRuntimeIdentifiersTargetFrameworks Include="$(TargetFrameworks)" />
+      <_AllRuntimeIdentifiers Include="$(RuntimeIdentifiers);$(RuntimeIdentifier)" />
+    </ItemGroup>
+
+    <MSBuild Projects="$(MSBuildProjectFile)"
+             Condition="'$(TargetFrameworks)' != ''"
+             Targets="GetAllRuntimeIdentifiers"
+             Properties="TargetFramework=%(_GetAllRuntimeIdentifiersTargetFrameworks.Identity)">
+      <Output ItemName="_AllRuntimeIdentifiers" TaskParameter="TargetOutputs" />
+    </MSBuild>
+
+    <PropertyGroup>
+      <RuntimeIdentifiers>@(_AllRuntimeIdentifiers->Distinct())</RuntimeIdentifiers>
+    </PropertyGroup>
+  </Target>
+
 </Project>

test/Microsoft.NET.Build.Tests/GivenThatWeWantToBuildADesktopExe.cs

@@ -173,6 +173,33 @@ public void It_appends_rid_to_outdir_correctly(string identifier, string rid, bo
             }
         }
 
+        [Fact]
+        public void It_builds_multitargeted_with_default_rid()
+        {
+            if (!RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+            {
+                return;
+            }
+
+            var testAsset = _testAssetsManager
+                .CopyTestAsset("HelloWorld")
+                .WithSource()
+                .WithProjectChanges(project =>
+                {
+                    var ns = project.Root.Name.Namespace;
+                    var propertyGroup = project.Root.Elements(ns + "PropertyGroup").First();
+                    propertyGroup.Element(ns + "TargetFramework").Remove();
+                    propertyGroup.Add(new XElement(ns + "TargetFrameworks", "net46;netcoreapp1.1"));
+                })
+                .Restore();
+
+            var buildCommand = new BuildCommand(Stage0MSBuild, testAsset.TestRoot);
+            buildCommand
+                .Execute()
+                .Should()
+                .Pass();
+        }
+
         [Theory]
         [InlineData("win7-x86", "x86")]
         [InlineData("win8-x86-aot", "x86")]