This isn't really backwards-incompatible, because false and nil are handled the same way in Lua, typically with

if (not something) then

So existing code will still behave the same. The chance that anyone did something like:

local b = isGuestAccount(acc)
if (type(b) == 'boolean') then

...is minimal

So existing code will still behave the same. The chance that anyone did something like:

local b = isGuestAccount(acc)
if (type(b) == 'boolean') then

...is minimal

I've seen cases of where people will do something like;

local b = isGuestAccount(acc)
if  b == false then

So the backwards incompatibility does apply here.

Thanks for this pull request, but even with the upcoming 1.7 release, I don't think we should touch the return value. It's just not worth it, for correctness sake.