C# 9.0 covariant return types break the engine, aboring running the entire test class that contains such code

[Rekkonnect]

I was trying to run tests for one class in my library, and it suddenly threw that issue in a class:

OneTimeSetUp: System.TypeLoadException : Return type in method '<type>.<function>()' on type '<type>' from assembly <assembly> is not compatible with base type method '<baseType>.<function>()'.

I was using NUnit 3.12.0, updated to 3.13.0, and now the tests will not even show a result; I'm assuming the issue silently occurs.

Basically, this issue applies to code of the following pattern:

class BaseClass
{
    virtual BaseClass Function();
}
class InheritingClass
{
    override InheritingClass Function();
}

For the specific case that I encountered, look at this commit's description.

[mikkelbu]

@alfasgd It sounds similar to dotnet/runtime#45344 (and related issues). Can you provide a small test samples that reproduces the problem?

[Rekkonnect]

Yes indeed this is a .NET 5.0 issue; current issue is dotnet/runtime#47007. Closing here.