Add .NET Standard compatibility warnings

Directory.Build.targets

@@ -36,6 +36,7 @@
   </ItemGroup>
 
   <Import Project="Sdk.targets" Sdk="Microsoft.DotNet.Arcade.Sdk" />
+  <Import Project="$(RepositoryEngineeringDir)packaging.targets" Condition="'$(IsPackable)' == 'true'" />
 
   <!-- Put it after Sdk.targets imports to override values from StrongName.targets. It's needed to avoid NGEN validation issues on VS exp insertions.  -->
   <PropertyGroup Condition="'$(SignType)' == 'test'">

eng/packaging.targets

@@ -0,0 +1,60 @@
+<Project>
+
+  <PropertyGroup>
+    <BeforePack>$(BeforePack);AddNETStandardCompatErrorFileForPackaging</BeforePack>
+    <PlaceholderFile>$(RepositoryEngineeringDir)_._</PlaceholderFile>
+  </PropertyGroup>
+
+  <!-- Include a netstandard compat error if the project targets .NETStandard and runtimes.
+       This prohibits users to consume packages on an older .NETCoreApp version than the minimum supported one. -->
+  <ItemGroup>
+    <NETStandardCompatError Include="netcoreapp2.0"
+                            Supported="$(LatestDotNetCoreForMSBuild)"
+                            Condition="$(TargetFrameworks.Contains('netstandard2.')) and $(TargetFrameworks.Contains('$(LatestDotNetCoreForMSBuild)'))" />
+    <NETStandardCompatError Include="net461"
+                            Supported="$(FullFrameworkTFM)"
+                            Condition="$(TargetFrameworks.Contains('netstandard2.0')) and $(TargetFrameworks.Contains('$(FullFrameworkTFM)')) and !$(TargetFrameworks.Contains('net35'))" />
+  </ItemGroup>
+
+  <!-- Add targets file that marks a .NETStandard applicable tfm as unsupported. -->
+  <Target Name="AddNETStandardCompatErrorFileForPackaging"
+          Condition="'@(NETStandardCompatError)' != '' and '$(DisableNETStandardCompatErrors)' != 'true'"
+          Inputs="%(NETStandardCompatError.Identity)"
+          Outputs="unused">
+    <PropertyGroup>
+      <_NETStandardCompatErrorFilePath>$(BaseIntermediateOutputPath)netstandardcompaterror_%(NETStandardCompatError.Identity).targets</_NETStandardCompatErrorFilePath>
+      <_NETStandardCompatErrorFileTarget>NETStandardCompatError_$(PackageId.Replace('.', '_'))_$([System.String]::new('%(NETStandardCompatError.Supported)').Replace('.', '_'))</_NETStandardCompatErrorFileTarget>
+      <_NETStandardCompatErrorFileContent>
+<![CDATA[<Project InitialTargets="$(_NETStandardCompatErrorFileTarget)">
+  <Target Name="$(_NETStandardCompatErrorFileTarget)"
+          Condition="'%24(SuppressTfmSupportBuildWarnings)' == ''">
+    <Warning Text="$(PackageId) $(PackageVersion) doesn't support %24(TargetFramework) and has not been tested with it. Consider upgrading your TargetFramework to %(NETStandardCompatError.Supported) or later. You may also set &lt%3BSuppressTfmSupportBuildWarnings&gt%3Btrue&lt%3B/SuppressTfmSupportBuildWarnings&gt%3B in the project file to ignore this warning and attempt to run in this unsupported configuration at your own risk." />
+  </Target>
+</Project>]]>
+      </_NETStandardCompatErrorFileContent>
+      <_NETStandardCompatErrorPlaceholderFilePackagePath>buildTransitive$([System.IO.Path]::DirectorySeparatorChar)%(NETStandardCompatError.Supported)</_NETStandardCompatErrorPlaceholderFilePackagePath>
+    </PropertyGroup>
+
+    <WriteLinesToFile File="$(_NETStandardCompatErrorFilePath)"
+                      Lines="$(_NETStandardCompatErrorFileContent)"
+                      Overwrite="true"
+                      WriteOnlyWhenDifferent="true" />
+
+    <ItemGroup>
+      <_PackageBuildFile Include="@(None->Metadata('PackagePath'));
+                                  @(Content->Metadata('PackagePath'))" />
+      <_PackageBuildFile PackagePathWithoutFilename="$([System.IO.Path]::GetDirectoryName('%(Identity)'))" />
+
+      <None Include="$(_NETStandardCompatErrorFilePath)"
+            PackagePath="buildTransitive\%(NETStandardCompatError.Identity)\$(PackageId).targets"
+            Pack="true" />
+      <!-- Add the placeholder file to the supported target framework buildTransitive folder, if it's empty. -->
+      <None Include="$(PlaceholderFile)"
+            PackagePath="$(_NETStandardCompatErrorPlaceholderFilePackagePath)\"
+            Pack="true"
+            Condition="'@(_PackageBuildFile)' == '' or
+                       !@(_PackageBuildFile->AnyHaveMetadataValue('PackagePathWithoutFilename', '$(_NETStandardCompatErrorPlaceholderFilePackagePath)'))" />
+    </ItemGroup>
+  </Target>
+
+</Project>

src/MSBuild/MSBuild.csproj

@@ -48,6 +48,7 @@
     <IncludeSatelliteOutputInPack>false</IncludeSatelliteOutputInPack>
     <NoPackageAnalysis>true</NoPackageAnalysis>
     <AddAppConfigToBuildOutputs>false</AddAppConfigToBuildOutputs>
+    <DisableNETStandardCompatErrors>true</DisableNETStandardCompatErrors>
 
     <!-- Setting DebugType here goes hand in hand with eng\AfterSigning.targets. This is to prompt the x64 build to produce a 'full' .pdb that's `more compatible` then 'portable' and 'embedded' .pdbs. This doesn't get set on 32 bit architecture, which will default to 'embedded' and 'pdb2pdb' will convert those as needed. See https://github.com/dotnet/msbuild/pull/5070 for context. -->
     <DebugType Condition="'$(Platform)' == 'x64'">full</DebugType>
@@ -319,4 +320,4 @@
   </Target>
 
 </Project>
-
+