Evaluation of null values and type checks - why is this an error?

Yeah, I think this is a known issue/limitation with type narrowing.

You may have to rewrite your code to use an if-statement instead. e.g.

var elapsedTime = 0.0

if (info.timerTime != null){
  elapsedTime = (info.timerTime - workoutStartTime)/1000.0;
}

In another life I program in Swift, so I know (and actually like) strict type checking. However, this is a bit annoying, so I will rather switch it off until the compiler has become a bit more intelligent.

In VS code, in the extension settings you can turn it off globally, or with the 4.1.7 compiler, you can turn it off on a per-project basis by including this in your monkey jungle file:

project.typecheck = 0

Yep, thanks, done that.

there are a few bug reports / feature requests about this, feel free to up-vote them:

https://forums.garmin.com/developer/connect-iq/i/bug-reports/language-feature-null-coalescing-operator-and-optional-chaining
https://forums.garmin.com/developer/connect-iq/i/bug-reports/type-checker-fails-to-remove-null-in-some-cases (this one is "being investigated" for 9 months...)

The type checker only claims to do data flow analysis on local variables (it *does* do it on non locals in some circumstances, but not very well).

From your snippet, I can't tell if workoutStartTime is a local - but the error message suggests that the type checker *does* know it's a number, so I'm guessing its a local.

Otoh info.timerTime certainly isn't a local. So I'm pretty sure that

if (workoutStartTime != null && workoutStartDistance != null) {
    var info_timerTime = info.timerTime;
    var elapsedTime = info_timerTime != null ? (info_timerTime - workoutStartTime)/1000.0 : 0.0;
    [...]
}

will work. Conveniently, it also generates smaller code (and assuming info.timerTime is almost always non-null, it will be faster code too).

I just came across this issue and rather than disabling the typing (which is there for a reason I created and used this `nullNumber` variable instead of `null`:

 var nullNumber = null as Number | Null;

This doesn't do what you think it does. If this is a class variable then you might mean:

var nullNumber as Number?; // it's not necessary, but if you want you can explicitly assign null to it: var nullNumber as Number? = null;

If it is a local variable, then forget about the "as ..." part, it doesn't do anything.

In addition to what flocsy🤠 wrote, I'm curious: how would you use this to solve the problem?

Changing all the "null"s in the original code to nullNumber certainly doesn't solve the issue - it's likely to make it worse; so where exactly are you going with this?