Flow analysis. Type of interest and the for loop increment part

[sgrekhov]

@stereotype441 is the following expected? Why if type T is made a type of interest in the increment part of the for(;;) loop then a variable of type S cannot be promoted to T in the body of the loop but can be after it?

class S {}

class T extends S {
  int answer() => 42;
}

test1() {
  S s = S();
  for (int i = 0; i < 1; s is T ? true : false, i++) {
    s = T();
    s.answer(); // Error. The method 'answer' isn't defined for the type 'S'. 
  }
}

test2() {
  S s = S();
  for (int i = 0; i < 1; s is T ? true : false, i++) {
  }
  s = T();
  s.answer(); // Ok
}

Dart SDK version: 3.9.0-23.0.dev (dev) (Tue Apr 15 21:06:42 2025 -0700) on "windows_x64"

[stereotype441]

Yes, this is working as intended. The reason is because the order in which flow analysis visits the code corresponds to the order in which it executes. Even though the "updater" part of a for(;;) loop appears textually before the loop body, it is always executed after the body. So flow analysis visits it after visiting the body. This means that if T is made a type of interest in the updater, that doesn't affect the body. But if T is made a type of interest in the body, that will affect the updater.

Closing as "not planned" since this is working as intended.

[sgrekhov]

Thank you!