String argument with optional value in std.getopt

[Hassan]

Hello

I'm trying to get getopt to recognize an argument that may or may 
not take a value. Here's an example :

./hashtrack --list
./hashtrack --list filter

The problem is that if I point list to a string variable, the 
first call fails with "Missing value for argument --list".

I tried having it point to a callback with an optional second 
parameter :

void cb(string option, string value = "")
{
     writeln("--list");
     option.writeln();
     value.writeln();
}

But I get the same error. I also tried having two callbacks with 
the same name. The first one takes a single argument, and the 
second one accepts two :

void cb(string option)
{
     writeln("--list");
     option.writeln();
}

void cb(string option, string value)
{
     writeln("--list");
     option.writeln();
     value.writeln();
}

But it only calls the first one. I'm using DMD64 D Compiler 
v2.090.0. Any pointers ?