Nullability promotion after bool field access in an if with a coalesce

[frmrgr]

It would be great if the analyzer was able to promote a type after a bool is accessed on a nullable reference and defaulted.

Example:

    void foo(String? s) {
      if (s?.isNotEmpty ?? false) {
-       print(s!.substring(0, 2));
+       print(s.substring(0, 2));
      }
    }

It can already handle longer version well:

    void foo(String? s) {
      if (s != null ? s.isNotEmpty : false) {
       print(s.substring(0, 2));
      }
    }

Not very important, but would be nice.

Dart SDK 2.17.1

[eernstg]

I don't think this is a duplicate of #1224, but there is definitely a lot of similarity. I don't know if it would be useful to somehow merge the two.

One thing that makes this issue different (and tricky) is that it depends of knowing that s?.isNotEmpty must have evaluated to true if the whole if-null expression s?.isNotEmpty ?? false yields true, and that's a property of _ ?? false which is not shared by an expression of the form _ ?? b where b is a boolean expression that turns out to yield false at run-time. We must be able to prove that the value is false already at compile time. So would we expect s?.isNotEmpty ?? (v && !v) to promote s when v is a local variable of type bool (because v && !v is false when v is false and also when v is true)? How much magic would we expect to have, in order to determine that the right operand of ?? yields false? It might work just fine to recognize the literal false and only that, but it may then seem silly that s?.isNotEmpty ?? (false) "doesn't work".

[frmrgr]

I think it's a dupe, sorry I hadn't found the earlier one.

Regarding the comment, I would not expect the analyzer to do algebra and it is good that if (s != null ? s.isNotEmpty : (v && !v)) doesn't work. I included if (s != null ? s.isNotEmpty : false) in the OP because I assume that what ?? is a syntactic sugar for. But that all has been addressed in the ticket you linked.