Configuring two returns on the same generic method with inheritance-related type arguments throws InvalidCastException

[OllieOptix]

Describe the bug
When two returns are configured on the same generic method, and the two closed type arguments are related by inheritance, configuring the second call throws an InvalidCastException. NSubstitute appears to hand back the value stored for the first configuration while recording the second call, then fails to cast it to the second call's return type.

This works in 5.3.0 and breaks in 6.0.0.

This was found with MediatR's IMediator.Send<TResponse>(IRequest<TResponse>), where different request types resolve TResponse to IResult vs IResult<T>.

To Reproduce
Create and run this console application. When NSubstitute is on version 5.3.0 the application writes "No Errors" to the console. When NSubstitute is on version 6.0.0 the application throws a System.InvalidCastException.

program.cs

using NSubstitute;

var sub = Substitute.For<ISender>();

sub.Send(Arg.Any<NonGenericRequest>()).Returns(Substitute.For<IResult>());
sub.Send(Arg.Any<GenericRequest>()).Returns(Substitute.For<IResult<int>>());
Console.WriteLine("No errors");

public interface IResult { }
public interface IResult<T> : IResult { }

public interface IRequest<TResponse> { }

public class NonGenericRequest : IRequest<IResult> { }
public class GenericRequest : IRequest<IResult<int>> { }

public interface ISender
{
    Task<TResponse> Send<TResponse>(IRequest<TResponse> request);
}

csproj

<Project Sdk="Microsoft.NET.Sdk">

    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net10.0</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
    </PropertyGroup>

    <ItemGroup>
<!--        <PackageReference Include="NSubstitute" Version="6.0.0" />-->
        <PackageReference Include="NSubstitute" Version="5.3.0" />
    </ItemGroup>

</Project>

Expected behaviour
I would expect this to behave like 5.3.0 and not throw an exception.

Environment:

• NSubstitute version: 6.0.0
• Platform: net10 project on Windows

Workaround

Use Arg.Is so it doesn't match the null recording placeholder:

sub.Send(Arg.Is<NonGenericRequest>(r => r != null)).Returns(Substitute.For<IResult>());
sub.Send(Arg.Is<GenericRequest>(r => r != null)).Returns(Substitute.For<IResult<int>>());

[Romfos]

this bug is related to this PR #862

@JMolenkamp could you please check your changes?

cc @dtchepak should we revert and release hotfix as temporary solution?

[dtchepak]

@Romfos should we revert this and the #856 nullability change causing #973 for now and can revisit in future?

[Romfos]

@dtchepak yes, lets do it

Options #2 is to revert #862 and merge #975 (comment)

[JMolenkamp]

@Romfos A very rough fix here. It works, but is probably not acceptable yet. I'll come back to this, probably next week.

It comes down to the equivalency of the generic arguments of the return types. Generally speaking, two types should not be considered equivalent when their generic arguments differ. So, the current fix is to exclude the IsAssignable check when checking generic arguments for return types.

[JSparshottO2]

How is this fix going? We're being held back in some test cases because of this. Any idea on an ETA?

[JMolenkamp]

#977
Has been there for several weeks already, don't know what else I should have done.