Code shared from run.dlang.io. Run with '-unittest'

main.d

import std.stdio;

void main()
{
    import std.parallelism;
    
    auto   a   = new A();
    auto   fn  = &a.download;
    string url = "https://raw.githubusercontent.com/dlang/dmd/master/VERSION";

    auto async_task = task!fn( url );  // error

    async_task.executeInNewThread();

    writeln("OK");
}


class A
{
    void download( string url )
    {
        // DO
    }
}

remove this