#nullable enable

using System;

​

public class Program

{

    public static void Main()

    {

        // works

        Func<bool, bool?> processor = x => !x;

        bool a = Process(false, processor);

        // works

        bool b = Process<bool, bool>(false, x => !x);

        // doesn't compile

        bool c = Process(false, x => !x);

        // these must also keep working

        Program d = Process(new Program(), x => x);

        bool e = Process<bool, bool>(false, x => x ? x : null);

        bool f = Process(false, x => x ? x : null);

    }

​

    public static TResult Process<TInput, TResult>(TInput input, Func<TInput, TResult?> processor)

        where TResult : struct

    {

        return processor(input) ?? throw new Exception();

    }

    public static TResult Process<TInput, TResult>(TInput input, Func<TInput, TResult?> processor)

        where TResult : class

    {

        return processor(input) ?? throw new Exception();

    }

    //public static TResult Process<TInput, TResult>(TInput input, Func<TInput, TResult?> processor)

    //  where TResult : notnull

    //{

    //  return processor(input) ?? throw new Exception();

    //}

    //public static TResult Process<TInput, TResult>(TInput input, Func<TInput, TResult> processor)

    //  where TResult : struct

    //{

    //  return processor(input);

    //}

}