@required seemingly ignored in default constructors

[duzenko]

• Dart SDK Version (dart --version)
  Dart VM version: 2.3.0-dev.0.5.flutter-a1668566e5 (Tue Apr 30 20:35:41 2019 +0200) on "windows_x64"
• Whether you are using Windows, MacOSX, or Linux (if applicable)
  Windows 10 x64
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  Flutter

class BasePage extends StatelessWidget {
  final String title;

  BasePage({@required this.title});
...
}

class OpportunitiesPage extends BasePage {
// default (no) constructor
...
}
...
OpportunitiesPage();

This seems to compile just fine to me. When debugger stops in BasePage() due to a breakpoint I can see title be null.
Is this the expected behavior?

[srawlins]

Ooh interesting catch. I've filed this as an enhancement request rather than a bug, because the specification of @required does not spell out that implicit no-arg super constructor calls should be included... if this is a valid request, it would probably be a new analyzer Hint, with specific text about the implicit call.

CC @bwilkerson

[srawlins]

CC @stereotype441 as well; I think this brings up Non-nullable type implications. It's interesting that BasePage({@required this.title}) is a valid "zero-arg constructor," when it accepts a named argument tied to a final field. I suppose it's okie dokey because the value of this.title will be assigned a default value of null if the constructor is called implicitly. NNBD features truly required named parameters, right?

The message on a new Hint regarding this implicit super call of a constructor with a @required parameter would say something like "You think you have a zero-arg super constructor, but as per the rules of @required, you don't. You must explicitly call the super constructor, or make all parameters of the default super constructor truly optional."

[bwilkerson]

Yes, NNBD defines truly required named parameters. In that world (assuming the annotation is replaced by the real syntax), the constructor in BasePage will no longer be a zero argument constructor.

I don't know whether it's worth adding a new hint (or enhancing the existing hint) to cover this case given that NNBD will define explicit support for this, but I'm happy to see it added if we think there's enough value.

[srawlins]

I just checked in tests for the @required filed formal parameter.

I'd like to close this feature request in favor of the same feature around the required keyword in the forthcoming non-nullable type system.

[srawlins]

This was previously reported at #30117; please follow along there.

[srawlins]

Duplicate of #30117