'not nil' check missing early function return.

[GrundleTrundle]

I'm getting a "can't prove o.p is not nil" error for the following test code:

type
   RefObj = ref object
   A = object
      p: RefObj

proc action(o1: RefObj not nil) =
  echo "action"

proc check(o: A) =
   if o.p == nil:
      return

   action(o.p)    # <-- can't prove o.p is nil

var
   x : A

check(x)

It doesn't seem to recognize that the control flow won't reach that statement if o.p == nil.

[Araq]

This is how the algorithm has been designed. It needs better documentation, but it is on par with the FP pattern matching solution so definitely good enough for version 1, IMO.

[wiml]

I just ran into this (I think) in Nim 1.2.6. This is a fairly frustrating defect — it makes it awkward to insert the manual nil checks at the boundaries between not nil and maybe nil code.

Although it might be a pain to track not-nil–ness through every kind of control flow, it seems to me that it should be straightforward for the compiler to understand the equivalence of

if condition:
   something-that-does-not-fall-through (like a return or raise)
more-statements

with

if condition:
   something-that-does-not-fall-through
else:
  more-statements

which would be good enough for quite a few use cases.