Allowing string value with getopt without requiring it

[Ithy]

Hello. I'm just getting into D (literally writing my first 
program), and I can't figure out how to do this with getopt:

I want to have the option --log, which will enable logging (set a 
bool variable to true). However, I want the program to also 
accept --log=filename to allow the user to define custom log 
file. If --log is used without filename, a default file will be 
used.

I tried this:

getopt(args,
     "log", &log,
     "log", &logFile);

Passing just --log to the program works fine, the boolean is set 
to true. However, using --log=test.log throws an exception, and 
seemingly getopt expects either true or false.

How would I go about implementing this without creating another 
option like --log-file for custom file name?