Merge pull request #2930 from eerhardt/MvcNet46

dotnet build raises errors MVC apps target net46.

TestAssets/TestProjects/AppWithNet46AndRoslyn/.noautobuild

@@ -0,0 +1 @@
+noautobuild

TestAssets/TestProjects/AppWithNet46AndRoslyn/Program.cs

@@ -0,0 +1,10 @@
+namespace AppWithNet46AndRoslyn
+{
+    public class MyApp
+    {
+        public static void Main()
+        {
+            System.Console.WriteLine("Hello, World!");
+        }
+    }
+}

TestAssets/TestProjects/AppWithNet46AndRoslyn/project.json

@@ -0,0 +1,11 @@
+{
+  "buildOptions": {
+    "emitEntryPoint": true
+  },
+  "dependencies": {
+    "Microsoft.CodeAnalysis.CSharp": "1.3.0-beta1-20160429-01"
+  },
+  "frameworks": {
+    "net46": { }
+  }
+}

src/Microsoft.DotNet.ProjectModel/ProjectContextBuilder.cs

@@ -440,15 +440,21 @@ namespace Microsoft.DotNet.ProjectModel
                     package.HasCompileTimePlaceholder &&
                     !TargetFramework.IsPackageBased)
                 {
+                    // requiresFrameworkAssemblies is true whenever we find a CompileTimePlaceholder in a non-package based framework, even if
+                    // the reference is unresolved. This ensures the best error experience when someone is building on a machine without
+                    // the target framework installed.
+                    requiresFrameworkAssemblies = true;
+
                     var newKey = new LibraryKey(library.Identity.Name, LibraryType.ReferenceAssembly);
                     var dependency = new LibraryRange(library.Identity.Name, LibraryType.ReferenceAssembly);
 
-                    // If the framework assembly can't be resolved then mark it as unresolved but still replace the package
-                    // dependency
-                    var replacement = referenceAssemblyDependencyResolver.GetDescription(dependency, TargetFramework) ??
-                                        UnresolvedDependencyProvider.GetDescription(dependency, TargetFramework);
-
-                    requiresFrameworkAssemblies = true;
+                    var replacement = referenceAssemblyDependencyResolver.GetDescription(dependency, TargetFramework);
+                    
+                    // If the reference is unresolved, just skip it.  Don't replace the package dependency
+                    if (replacement == null)
+                    {
+                        continue;
+                    }
 
                     // Remove the original package reference
                     libraries.Remove(pair.Key);

test/dotnet-build.Tests/GivenDotnetBuildBuildsProjects.cs

@@ -100,5 +100,20 @@ namespace Microsoft.DotNet.Tools.Builder.Tests
                 "TestLibraryWithXmlDoc.xml"
             });
         }
+
+        [WindowsOnlyFact]
+        public void It_builds_projects_targeting_net46_and_Roslyn()
+        {
+            var testInstance = TestAssetsManager
+                .CreateTestInstance("AppWithNet46AndRoslyn")
+                .WithLockFiles();
+
+            var testProject = Path.Combine(testInstance.TestRoot, "project.json");
+
+            new BuildCommand(testProject)
+                .Execute()
+                .Should()
+                .Pass();
+        }
     }
 }