[lmm]

If you want to be able to return "pointer or null", any decent language will let you do that, sure. But it's good to have the option of saying "this function will never return null".

Think of it this way: functions should document whether (and under what conditions) they return null, right? What if the compiler could check the accuracy of that documentation, so it would be an error to return null from a function that said it didn't return null, and a warning to document a function as possibly-returning-null when it never did? (And once you had that, surely you'd want a warning when you accessed a possibly-null thing without checking whether it was actually null?)

[Tloewald]

So basically you want the compiler to solve the halting problem for every function it compiles.

[lmm]

Ideally yes. But keeping track of nullability is not impossible, it's not even hard, as any number of existing languages with such checking prove (some of which have very performant compilers).