On Tuesday, 8 May 2018 at 22:31:10 UTC, Yuxuan Shui wrote: > snip] This doesn't compile for me on run.dlang.io: onlineapp.d(22): Error: template onlineapp.f cannot deduce function from argument types !()(B), candidates are: onlineapp.d(1): onlineapp.f(T)(immutable T a)