Type propagation for final fields

[renggli]

I think the type system is too conservative for final fields:

class Foo {
  final Object object;

  Foo(this.object);

  void bar() {
    if (object is String) {
      print(object.length);  // The getter 'length' isn't defined for the type 'Object'. 
    }
  }
}

Similarly for null type promotion:

class Bar {
  final String? message;

  Bar([this.message]);

  void bar() {
    if (message != null) {
      print(message.length);  // An expression whose value can be 'null' must be null-checked before it can be dereferenced.
    }
  }
}

Having to copy to a local variable before testing and accessing is cumbersome.

$ dart --version
Dart SDK version: 2.10.0-92.0.dev (dev) (Wed Sep 2 22:28:32 2020 -0700) on "macos_x64"

[mraleph]

Final fields can still be overriden by a getter making any sort of static promotion without checks unsound.

class Bar extends Foo {
  @override
  Object get object => 
    Random.nextBool() ? 'today I am a string, some day I am ' : 42 ;
}

There are multiple threads going on about potential ways to tackle this on the language repo:

• Provide a "forced promotion" operator with runtime checking. language#1187
• Proposal: context-gated field promotion with runtime checking language#1188
• Binding type cast and type check. language#1191
• If-variables language#1201