+0 from me.

Another use is computing the Carmichael function for composite numbers (like an RSA encryption modulus, in which context the Carmichael function is routinely used).

But only +0 instead of +1 because it's so easy to build from gcd().

I don't agree it's tricky at all.  While lcm(0, 0) undoubtedly should return 0 in a general-purpose library function, in my own code I've never supplied that.  Because in every application I've ever had for it, I would rather get an exception if I ever passed two zeroes - that would always have been a mistake.