enableing nullable checking project wide breaks generic usercontrols (CS0263)

• .NET Core Version: 5.0.403 bf9d168ac2 SDK (core 3.1)
• Windows version: Win10 | 20H2 Build 19042.1052
• Does the bug reproduce also in WPF for .NET Framework 4.8?: have not checked

Problem description: Assume this minimal test project:

namespace WpfApp1.Tests {
  public class GridBase : UserControl { }

  public class CCCCCC<T> : GridBase where T : class { }

  public class testclass { }

  public partial class UserControl1 : CCCCCC<testclass> {
    public UserControl1() {
      InitializeComponent();
    }
  }
}

<local:CCCCCC x:Class="WpfApp1.Tests.UserControl1"
        x:TypeArguments="local:testclass"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
        xmlns:local="clr-namespace:WpfApp1.Tests"
        mc:Ignorable="d" 
        d:DesignHeight="450" d:DesignWidth="800">
    <DataGrid Background="BlueViolet">
    </DataGrid>
</local:CCCCCC>

This UserControl compiles fine with the following project settings:

<Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">
 <PropertyGroup>
   <OutputType>WinExe</OutputType>
   <TargetFramework>netcoreapp3.1</TargetFramework>
   <UseWPF>true</UseWPF>
   <!--<Nullable>enable</Nullable>-->
 </PropertyGroup>
</Project>

but once you uncomment the project wide nullable setting, the whole project no longer compiles and the following error occurs; CS0263 Partial declarations of 'UserControl1' must not specify different base classes WpfApp1 [...]\WpfApp1\UserControl1.xaml.cs 18

This however is a different error message from the one you actually get when using two different base classes for your xaml and c# side (usually you get Base class of 'WpfApp1.Tests.UserControl1' differs from declared in other parts).

[SaculRennorb]

also: a workaround is:

#nullable disable
  public partial class UserControl1 : CCCCCC<testclass> {
#nullable enable

[miloush]

@kkirkfield compiles fine for me one 7.0.100-rc.1.22431.12 and VS 17.4 preview. As noted above, this is not a WPF but Roslyn issue. Note the https://github.com/dotnet/roslyn/issues/63490 is on 17.4 milestone.