analyzer complains even when checking for null values

[MohiuddinM]

I get an error even when checking for null value:
An expression whose value can be 'null' must be null-checked before it can be dereferenced at

class T {
  final String? s;

  T(this.s);

  void f() {
    if (s != null && s.length > 3) {}
  }
}

[lrhn]

The reason that a test for null can sometimes cause a later use of a nullable variable to be allowed, is that it causes a type promotion from the nullable type to the non-nullable (sub-)type. This is the same as any other type promotion, say having a num and checking that it's an int.

Dart only performs type promotion when it's guaranteed to be safe, when the compiler is absolutely certain that the value you check is also the value you use later.
That currently means that you can only promote local variables. Even a private field might be overridden by a getter in a subclass, so the compiler cannot locally determine for sure that the value won't change between check and use.

You can copy the variable to a local variable:

void f() {
  var s = this.s;
  if (s != null && s.length > 3) { /*...*/ }
}

or you can add a null check to the length access:

void f() {
  if (s != null && s!.length > 3) { /*...*/ }
}

[lrhn]

Could the analyzer give a different error message when something looks like the user wanted it to promote, but it doesn't for non-local variables.

[eernstg]

@stereotype441, would it be overkill to let the analyzer maintain the same information about non-local variables (that is, instance and static variables of the enclosing class and top-level variables) as the information which is currently maintained for local variables, and then check for each failing member lookup whether the member is statically known based on the would-be promoted type? We could then report that the failure could be caused by a lack of promotion.

[stereotype441]

@eernstg not at all. We have been talking about similar ideas for a while, just haven't gotten time to work on it. Main tracking issue is here: #38773

[srawlins]

Follow #38773 for the resolution to this issue.