Basically, need nullptr_t or the like to actually work. Henri ended up hacking around this in 792790, and I'm having to do the same in bug 779048 (where I have ambiguous overloads between "void*" and "XMLHttpRequest*" when nullptr is passed, plus a non-matching "template<typename T>" that takes a "T*"). It would be nice if this typedefery just lived in mfbt.

I think this needs us to implement the nullptr replacement for compilers which don't support it as suggested in bug 626472 comment 1.

Just use decltype(nullptr), now that we no longer emulate nullptr anywhere.