Error final field must be initialized in constructor even though null is the default

[terrylucas]

class X {
  final num a;

  const X([this.a]);

  const X.constant(); // Error "The final field a must be initialized"
}

If I change the X.constant constructor to be :

     const X.constant() : this();

then it works.

Seems a little wierd that a is already assigned to null shouldn't it just do that. If I try to assign a outside of the initializer list then I'd expect an error can't change a final.

[kasperl]

Correct me if I'm wrong Terry, but it looks like this is VM issue not a dart2js issue. I've changed the area label accordingly.

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Removed Area-Dart2JS label.
Added Area-VM, Triaged labels.

[iposva-google]

Set owner to @mhausner.
Added Accepted label.

[DartBot]

This comment was originally written by @mhausner

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I think the VM handles this correctly. The spec says:

Each final instance variable f declared in the immediately enclosing class must have an initializer in k's initializer list unless it has already been initialized by one of the following means:
 - Initialization at the declaration of f.
 - Initialization by means of an initializing formal of k.
or a compile-time error occurs.

Consider the class given above:

class X {
  final num a;
  const X([this.a]);
  const X.constant(); // Error "The final field a must be initialized"
}

Final instance field a does not have an initializer at the declaration. Thus, it must be initialized by the initializer list of the constructor.

Constructor X([this.a]) initializes a, either to an explicitly given value, or implicitly to null if no parameter is passed to X().

Constructor X.constant() however does NOT contain final field a in its initializer list, thus the VM correctly complains that a is not initialized.

If X.constant() redirects to X(), final field a is initialized, as if X() is called directly.

The VM handles all this correctly. Gilad confirms my understanding.

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Added Invalid label.