Flow typing for null checks: why not by default?

Scala Improvement Process
1

Under -Yexplicit-nulls, Scala currently employs a form of flow typing to enable code like this

def orZero(x: Int | Null): Int =
  if x != null then x else 0

In explicit nulls mode, the compiler determines that it’s OK to use x, which is of type Int | Null, in a position where Int is required, because the appropriate null check has been performed. But when -Yexplicit-nulls is not given, this fails to compile:

-- [E007] Type Mismatch Error: -------------------------------------------------
2 |  if x != null then x else 0
  |                    ^
  |                    Found:    (x : Int | Null)
  |                    Required: Int
  |

Why wouldn’t we always enable this?

10 Likes