Template instantiation fails on Linux, succeeds on Windows

[Bastiaan Veelo]

Hi!

The code in [1] compiles and runs flawlessly on Windows, but not 
on Linux (neither run.dlang nor Travis docker image). Any idea 
what can be done?

errors:
Error: undefined identifier __va_list_tag
onlineapp.d(282): Error: template instance 
`onlineapp.SetFactory!byte` error instantiating
Error: undefined identifier __va_list_tag
onlineapp.d(288): Error: template instance 
`onlineapp.SetFactory!(Count)` error instantiating
Error: undefined identifier __va_list_tag
onlineapp.d(290): Error: template instance 
`onlineapp.SetFactory!char` error instantiating

SetFactory is defined on row 233, and it seems that the problem 
relates to the vararg opIndex.

[1] https://run.dlang.io/gist/6dcbe4e82846cbeb3bf478e92a2b6e6f