Generics return type with explicit layout in PInvoke

[Enigmatrix]

Types with generic parameters and explicit layout should be able to be returned from PInvoke methods (assuming the type is blittable/unmanaged of course).

Currently on .NET Core 3.1, and it just silently fails

[Dotnet-GitSync-Bot]

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[Enigmatrix]

This is an example of the struct I am returning from a PInvoke function.

[StructLayout(LayoutKind.Sequential)]
public struct Error
{
    public uint Code;
    public IntPtr Cause;
}

[StructLayout(LayoutKind.Explicit)]
public struct FfiResult<T> where T: unmanaged
{
    [FieldOffset(0)] public long Tag;
    [FieldOffset(8)] public T Ok;
    [FieldOffset(8)] public Error Error;
}

This gives Could not load type 'FfiResult'1 from assembly '...' because generic types cannot have explicit layout

[jkoritzinsky]

I think this falls more under type system/vm than interop since the runtime blocks loading a generic type with explicit layout, so we don't even get to the point of failing to use it in interop scenarios.

[AaronRobinsonMSFT]

Closing as this is a duplicate of #43486.