No warning in case of variable assignment before the first type test

[sgrekhov]

class C {}
class D {}

main() {
  Object o = C();
  if (o is C) {} // No warning
  o = C();
  if (o is C) {} // unnecessary_type_check warning.
  o = D();
  if (o is D) {} // No warning
  if (o is D) {} // No warning
  o = D();
  if (o is D) {} // unnecessary_type_check warning.
}

Why there is no warning after the first assignment of the new object instance?

Dart SDK version: 3.7.0-183.0.dev (dev) (Fri Nov 22 20:03:46 2024 -0800) on "windows_x64"

[eernstg]

That's OK!

The promotion mechanism relies on 'types of interest', and the initialization of o with a value of type C doesn't make C a type of interest (it only makes a non-nullable type a type of interest in cases like int? iq = 2;). The same thing is true for assignments, they only promote the type of a variable if that type is already of interest for that variable for other reasons.

This means that o is C tests whether an o of type Object also has type C, which is OK (non-redundant).

But in the code that follows the first if statement, C is a type of interest for o, so o = C() promotes o to C, which makes the second type test redundant.

The cases testing o is D are similar: o is D turns D into a type of interest for o, and the second o = D() then promotes o. The first assignment o = D() does not promote o because D is not of interest for o at that time.