using System;

using System.Threading.Tasks;

public class Program

{

    public static void Main()

    {

        var a = Task

            .FromResult("foo")

            .Map(x => $"hello {x}"); // ERROR

​

        var b = Task

            .FromResult(1)

            .Map(x => x.ToString()); // ERROR

​

        var c = Task

            .FromResult(1)

            .Map(x => x + 1); // works

​

        var d = Task

            .FromResult("foo")

            .Map(x => x.Length); // works

​

        var e = Task

            .FromResult(1)

            .Map(_ => 0); // ERROR

​

        var f = Task

            .FromResult("foo")

            .Map<string, string>(x => $"hello {x}"); // works

​

        var g = Task

            .FromResult(1)

            .Map<int, int>(_ => 0); // works

​

        string foo(string x) => $"hello {x}";

        var h = Task

            .FromResult("foo")

            .Map(foo); // works

​

        var i = Task

            .FromResult("foo")

            .Map2(x => $"hello {x}"); // works

    }

}

​

public static class Ext {

    public static T2 Map<T1, T2>(this T1 x, Func<T1, T2> f) => f(x);

    public static async Task<T2> Map<T1, T2>(this Task<T1> x, Func<T1, T2> f) => (await x).Map(f);

}

​

public static class Ext2 {

    public static T Map2<T>(this T x, Func<T, T> f) => f(x);

    public static async Task<T> Map2<T>(this Task<T> x, Func<T, T> f) => (await x).Map2(f);

}

​