EmbedUntrackedSources causes WPF project build to fail when obj folder is redirected

[AArnott]

1. Create new WPF application in Visual Studio 2019 (.NET Core is what I chose)
2. Create a git repo around the solution.
3. In the solution directory, drop a Directory.Build.props file with this content to redirect the obj directory to a solution level (for a read only source tree):

<Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <BaseIntermediateOutputPath>$(MSBuildThisDirectory)\obj\$(MSBuildProjectName)\</BaseIntermediateOutputPath>
  </PropertyGroup>
</Project>

The build succeeds. Now add these elements to the Directory.Build.props file:

  <PropertyGroup>
    <EmbedUntrackedSources>true</EmbedUntrackedSources>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0-beta2-19554-01" PrivateAssets="All" />
  </ItemGroup>

Then build again.
The build will fail:

1>------ Rebuild All started: Project: WpfApp3, Configuration: Debug Any CPU ------
1>D:\obj\WpfApp3\Debug\netcoreapp3.0\MainWindow.g.cs(58,21,58,38): error CS1504: Source file 'MainWindow.xaml' could not be opened -- Could not find file.
1>D:\obj\WpfApp3\Debug\netcoreapp3.0\App.g.cs(51,21,51,31): error CS1504: Source file 'App.xaml' could not be opened -- Could not find file.
1>Done building project "WpfApp3.csproj" -- FAILED.
========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ==========

[tmat]

This is a bug in WPF: dotnet/wpf#1718

[AArnott]

Actually, this is still reproing for me, even when the 'fix' for dotnet/wpf#1718 is applied (by setting IncludePackageReferencesDuringMarkupCompilation=true). And anyway, that bug seems to focus on the markup compilation, whereas in my repro the markup compilation completes successfully. It's the subsequent csc in the original project that fails.