.razor components produce an unfixable warning when using @implements or @inherits with a generic type

[gulbanana]

Describe the bug

When Nullable Reference Types are enabled (<Nullable>enable</Nullable> in the .csproj), referring to a supertype with nullable parts results in a generated .razor.g.cs which disables nullable reference types then uses one. This line:
@implements SomeGeneric<T?>
results in warning CS8669, "The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. Auto-generated code requires an explicit '#nullable' directive in source.". However, there is no way to enable the nullable annotations context for the generated code or to disable it for the .razor file, leaving you in a catch-22 situation.

To Reproduce

Here is a simple repro: https://github.com/gulbanana-bugs/repro-blazor-nullable-generic-parameters

Further technical details

• ASP.NET Core version
  5.0.6

• Include the output of dotnet --info

$ dotnet --info
.NET SDK (reflecting any global.json):
 Version:   5.0.203
 Commit:    383637d63f

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.19042
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\5.0.203\

Host (useful for support):
  Version: 5.0.6
  Commit:  478b2f8c0e

.NET SDKs installed:
  5.0.103 [C:\Program Files\dotnet\sdk]
  5.0.201 [C:\Program Files\dotnet\sdk]
  5.0.203 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.15 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.15 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.15 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.6 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET runtimes or SDKs:
  https://aka.ms/dotnet-download

• The IDE (VS / VS Code/ VS4Mac) you're running on, and its version
  VS Code 1.56.2

Thanks for contacting us.

We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues.
To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.

[mkArtakMSFT]

@captainsafia do you remember what was the fix in the compiler which should have fixed this? We would like to confirm that it's fixed.

[captainsafia]

@mkArtakMSFT The external issue is dotnet/roslyn#45960.

[mkArtakMSFT]

Closing as the underlying roslyn issue has been fixed.