[analyzer] not show error when List generics type implicit promotion

[CoderBuck]

code:

class A {}
class A1 extends A {}
class A2 extends A {}

void main() {
  List<A> list = <A>[];
  list = <A1>[];  //   list type is changed to List<A1> 
  list.add(A2());
}

when run

> dart --version
Dart SDK version: 2.18.2 (stable) (Tue Sep 27 13:24:11 2022 +0200) on "macos_x64"

[mraleph]

Dart's generics are covariant - so this is working as intended.

Language team has previously discussed introducing variance annotations which would allow developers to control this. See

• Use-site variance
• Declaration-site variance

[eernstg]

I think it could be useful to mention one more thing:

class A {}
class A1 extends A {}
class A2 extends A {}

void main() {
  List<A> list = <A>[];
  list = <A1>[];  // `list.runtimeType == List<A1>`, static type still `List<A>`.
  list.add(A2()); // Accepted at compile time due to covariance, throws at run time.
}

The reason why list is not promoted at list = <A1>[] is that List<A1> is not a 'type of interest' for list at that point (or ever, in fact), and an assignment will only promote to a type of interest.

You can make a type a type of interest by checking for it:

void main() {
  List<A> list = <A>[];
  if (list is! List<A1>) list = <A1>[];  // Promote `list` to `List<A1>`.
  list.add(A2()); // Compile-time error.
}