[TestFixture]
public class ReproTest
{
   [Test]
   public void ResolveAllDecoratedCommandHandlers_IsSuccessful()
    {
        //Arrange
        var builder = new ContainerBuilder();

        builder.RegisterType(typeof(CommandHandler1))
            .As(typeof(ICommandHandler<Command>))
            .As(typeof(IHandler<Command>));
        builder.RegisterType(typeof(CommandHandler2))
            .As(typeof(ICommandHandler<Command>))
            .As(typeof(IHandler<Command>));

        builder.RegisterGenericDecorator(
            typeof(CommandHandlerDecorator<>),
            typeof(IHandler<>),
            context => context.ImplementationType.GetInterfaces().Any(t => t.IsGenericType 
                && t.GetGenericTypeDefinition() == typeof(ICommandHandler<>))
        );

        var container = builder.Build();

        //Act
        var commandHandlers = ((IEnumerable<IHandler<Command>>)container.Resolve(typeof(IEnumerable<IHandler<Command>>))).ToList();
        
        //Assert
        Assert.AreEqual(
            ((CommandHandlerDecorator<Command>)commandHandlers[0]).Decorated.GetType(), 
            typeof(CommandHandler1)); //fails, decorated is typeof(CommandHandler2)
        Assert.AreEqual(
            ((CommandHandlerDecorator<Command>)commandHandlers[1]).Decorated.GetType(), 
            typeof(CommandHandler2));

    }
}

interface IRequest { }
interface IHandler<in TRequest> where TRequest : IRequest { public void Handle(TRequest request); } 

interface ICommand: IRequest { }
interface ICommandHandler<in TCommand>: IHandler<TCommand> where TCommand: ICommand { } 

class Command : ICommand { }
class CommandHandler1 : ICommandHandler<Command> { public void Handle(Command command) => Console.WriteLine("CommandHandler1"); }
class CommandHandler2 : ICommandHandler<Command> { public void Handle(Command command) => Console.WriteLine("CommandHandler2"); }

class CommandHandlerDecorator<TCommand> : ICommandHandler<TCommand> where TCommand : ICommand
{
    public ICommandHandler<TCommand> Decorated { get; }
    public CommandHandlerDecorator(ICommandHandler<TCommand> decorated) => Decorated = decorated;
    public void Handle(TCommand request)
    {
        Console.WriteLine($"Command Decorator for {Decorated.GetType().FullName}");
        Decorated.Handle(request);
    }
}