[mono] UnsafeAccessor to a virtual method on a base class through a derived type asserts

[vitek-karas]

    class InheritanceBase
    {
        protected virtual string BaseVirtual() => $"{nameof(InheritanceBase)}.{nameof(BaseVirtual)}";
    }

    class InheritanceDerived : InheritanceBase
    {
    }

    [Fact]
    public static void Verify_InheritanceMethodResolution()
    {
        var instance = new InheritanceDerived();
        Assert.Equal($"{nameof(InheritanceBase)}.{nameof(BaseVirtual)}", BaseVirtual(instance));

        [UnsafeAccessor(UnsafeAccessorKind.Method, Name = nameof(BaseVirtual))]
        extern static string BaseVirtual(InheritanceDerived target);
    }

This fails with

* Assertion at F:\dotnet\runtime4\src\mono\mono\metadata\marshal-lightweight.c:2508, condition `target_method->klass == target_class' not met

@lambdageek - could you please route this to the right area/person

[vitek-karas]

Actually this fails on NativeAOT as well (at runtime!):

System.MissingMethodException: Method not found: 'BaseVirtual'.
   at Internal.Runtime.TypeLoaderExceptionHelper.CreateMissingMethodException(ExceptionStringID, String) + 0x63
   at Internal.Runtime.CompilerHelpers.ThrowHelpers.ThrowMissingMethodException(ExceptionStringID, String) + 0x25
   at UnsafeAccessorsTests.<Verify_InheritanceMethodResolution_BaseViaDerived>g__BaseVirtual|33_0(UnsafeAccessorsTests.InheritanceDerived target) + 0x1f
   at UnsafeAccessorsTests.Verify_InheritanceMethodResolution_BaseViaDerived() + 0x4b

CoreCLR works and the same code without UnsafeAccessor:

var instance = new InheritanceDerived();
instance.BaseVirtual();

will also work (probably on all runtimes, although I didn't try that).

@AaronRobinsonMSFT @MichalStrehovsky what do you think - should this work?

[MichalStrehovsky]

Well method BaseVirtual is not on InheritanceDerived so this is not expected to work.

This is what I brough up in #86932 (comment) and was rejected.

[AaronRobinsonMSFT]

@vitek-karas If I update as following it works, right?

        [UnsafeAccessor(UnsafeAccessorKind.Method, Name = nameof(BaseVirtual))]
        extern static string BaseVirtual(InheritanceBase target);

If so, I don't see why one would declare this as InheritanceDerived, when the target method is defined on a different type. It isn't as if I can know about non-public methods by looking at a derived type. The first thing I would do would be to look at InheritanceDerived, not see a non-public method and be rather confused.

[MichalStrehovsky]

Does this work with CoreCLR?

[vitek-karas]

Yes - it does work on CoreCLR.

[MichalStrehovsky]

Yes - it does work on CoreCLR.

@AaronRobinsonMSFT I think this should be in 8.0. We need to either block this in CoreCLR, or fix in NativeAOT/Mono.

[MichalStrehovsky]

Btw, I think the CoreCLR behavior is the correct one