Type test operator is works unintended for class final members

[kikuchy]

Problem

According to Language Tour, the combination of if statement and is operator can omit omit type cast by as operator in if statement.

But final ized class members cannot cast automatically.

class A {
  final Object a;
  A(this.a);
  void foo() {
    if (a is String) {
      print(a.codeUnits);    // Error: The getter 'codeUnits' isn't defined for the class 'dart.core::Object'.
    }
  }
}

final ized class members will not be exchanged for another instance, so its type will not change.

I think it should be compiled successfully.

Environment

• Dart SDK Version (dart --version)
  • Dart 2.1.0-dev.5.0.flutter-a2eb050044
• Whether you are using Windows, MacOSX, or Linux (if applicable)
  • MacOSX

[eernstg]

This is working as intended. It would be sound to extend the ability to promote a variable to the case where it is a final variable of any kind, but it is actually only supported for local variables (which includes formal parameters of an enclosing function). So it's definitely a useful enhancement (and we have proposals for enhancing promotion in various ways, but it will take a while before it's part of the language), but with the current rules there is no promotion of a in your example.

[eernstg]

Here's the place where you can see that promotion only occurs for local variables.

More information about the history here: #25565.

[kikuchy]

we have proposals for enhancing promotion in various ways, but it will take a while before it's part of the language

On, I see. I'm looking forward to that proposals becomes part of the language.
Thank you for answering.

[eernstg]

I'm looking forward to that proposals becomes part of the language.

Me too. ;-)