Merge pull request dotnet#699 from dsplaisted/remove-duplicate-package-check-before-restore

Don't run CheckForImplicitPackageReferenceOverrides before NuGet restore

Author: dsplaisted

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

@@ -93,7 +93,7 @@ Copyright (c) .NET Foundation. All rights reserved.
   <UsingTask TaskName="CheckForImplicitPackageReferenceOverrides" AssemblyFile="$(MicrosoftNETBuildTasksAssembly)" />
 
   <!-- Remove package references with metadata IsImplicitlyDefined = true, if there are other PackageReference items with the same identity -->
-  <Target Name="CheckForImplicitPackageReferenceOverrides" BeforeTargets="_GetRestoreProjectStyle;_CheckForInvalidConfigurationAndPlatform;CollectPackageReferences">
+  <Target Name="CheckForImplicitPackageReferenceOverrides" BeforeTargets="_CheckForInvalidConfigurationAndPlatform;CollectPackageReferences">
     <PropertyGroup>
       <ImplicitPackageReferenceInformationLink>https://aka.ms/sdkimplicitrefs</ImplicitPackageReferenceInformationLink>
     </PropertyGroup>
@@ -112,6 +112,41 @@ Copyright (c) .NET Foundation. All rights reserved.
 
   </Target>
 
+  <!-- Running an SDK task before the NuGet restore task causes issues when running on .NET Framework because it causes the
+      .NET Standard NuGet DLLs to be loaded from the SDK path rather than the .NET Framework versions from the NuGet targets
+      path.  To avoid this, we create a separate target to run before NuGet restore which deduplicates the items without
+      causing the SDK tasks to be loaded (but doesn't generate a warning message, because we need to load the tasks for that). -->
+  <PropertyGroup Condition="'$(DisableImplicitFrameworkReferences)' != 'true'">
+    <_ImplicitPackageName Condition="'$(TargetFrameworkIdentifier)' == '.NETStandard'">NETStandard.Library</_ImplicitPackageName>
+    <_ImplicitPackageName Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp'">Microsoft.NETCore.App</_ImplicitPackageName>
+  </PropertyGroup>
+
+  <ItemGroup Condition="'$(_ImplicitPackageName)' != ''">
+    <!-- Filter PackageReference to items where the ItemSpec matches the implicit package name, and add IsImplicitlyDefined metadata
+         for items that don't have it-->
+    <_ImplicitPackageReferenceCheck
+        Include="@(PackageReference->WithMetadataValue('Identity', '$(_ImplicitPackageName)'))">
+      <IsImplicitlyDefined Condition="'%(IsImplicitlyDefined)' != 'true' ">false</IsImplicitlyDefined>
+    </_ImplicitPackageReferenceCheck>
+
+    <!-- Now filter down to an item with just the implicit reference and another one with just the overriding reference -->
+    <_ImplicitPackageReference Include="@(_ImplicitPackageReferenceCheck->WithMetadataValue('IsImplicitlyDefined', 'true'))"/>
+    <_OverridingPackageReference Include="@(_ImplicitPackageReferenceCheck->WithMetadataValue('IsImplicitlyDefined', 'false'))"/>
+  </ItemGroup>
+
+  <Target Name="CheckForImplicitPackageReferenceOverridesBeforeRestore" BeforeTargets="_GetRestoreProjectStyle">
+    <ItemGroup>
+      <!-- Remove both the implicit and the override item, if there was both an implicit and an override item -->
+      <PackageReference Remove="@(PackageReference->WithMetadataValue('Identity', '$(_ImplicitPackageName)'))"
+                        Condition="'@(_ImplicitPackageReference)' != '' And '@(_OverridingPackageReference)' != ''"
+                      />
+
+      <!-- Add the override item back -->
+      <PackageReference Include="@(_OverridingPackageReference)"
+                        Condition="'@(_ImplicitPackageReference)' != '' And '@(_OverridingPackageReference)' != ''" />
+    </ItemGroup>
+  </Target>
+
   <UsingTask TaskName="CheckForDuplicateItems" AssemblyFile="$(MicrosoftNETBuildTasksAssembly)" />
 
   <Target Name="CheckForDuplicateItems" BeforeTargets="_CheckForInvalidConfigurationAndPlatform;CoreCompile">