iCloud restore "Enter Old Passcode"
I contacted Apple's technical support who said:


  At this point then we won’t be able to get past the passcode unless you manage to remember it at a later point in time
  
  The only way around this would be use the phone as new, and then sync with iCloud contacts, calendar and Photo library if you use those.


Personally, I have restored from iCloud and I was never prompted for my passcode so I guessed that it was prompting from a verification point-of-view, rather than an encryption point-of-view.

On an iPad, I enabled iCloud and signed in using the same Apple ID.

On the iPhone, restoring from iCloud prompted for a verification code from the iPad and succeeded.

Apple's technical support said:


  Oh perfect, you should be working here, haha