`malloc(3)`. Many of the functions in string.h.
My guess is this is usually true in practice, but in C and C++ dereferencing a null pointer is UB so you really can't assume that.
What I'm saying is there isn't an ergonomic issue with NULL error return APIs. You might prefer algebraic error types (in other languages). I'm not even sure I do. NULL return APIs are clean and simple. If you do (accidentally) forget to check for NULL return code, you will for sure get a segfault on first access in practice (not on embedded platforms maybe). The compiler can't remove any following code unless it can prove only NULL will ever be returned. Don't fall trap to UB FUD.
It can reorder code, though. For example, it can change
int *x = malloc(sizeof(int));
*x = 42;
if (x)
launch_the_nukes();
int *x = malloc(sizeof(int));
launch_the_nukes();
*x = 42;Most of the UB FUD are contrived examples that hardly happen in practice. What I'm saying is not that you should write bugs, but that bugs (especially segfaults) will immediately turn up in practice.