There is nothing to prevent the compiler from deducing that x is non-null without UB. What UB permits is for the the compiler to deduce that x is not null in the absence of information.

so : if(x)f(x) ...code ... if(x)g(x) doesn't need UB but y = x-> n ..... code if(x)g(x) - the compiler uses UB to remover the check. This will be especially horrible when LTO becomes more prevalent.