extension types not being recognised as objects

[jellynoone]

import 'dart:collection';

void main() {
  ['Hi', '', 'Hello'].map(FullString.fromOr).nonNulls; // The getter 'nonNulls' isn't defined for the type...
  <String?>['Hi', '', 'Hello'].nonNulls; // works as expected
}

extension type FullString._(String value) {
  static FullString? fromOr(String value) {
    if (value.isEmpty) return null;
    return FullString._(value);
  }
}

Invoking dart analyze main.dart produces the above error. However, I cannot reproduce this using dartpad so it might be something wrong with my installation.

Dart version: Dart SDK version: 3.8.1 (stable) (Wed May 28 00:47:25 2025 -0700) on "macos_arm64"

[eernstg]

[Edit: Corrected the example and the description.]

Checking the declaration of NullableIterableExtensions, I can see that nonNulls has on-type Iterable<T?> where T extends Object. But FullString isn't a subtype of Object, so we can't just jump ahead and choose T == FullString.

One way to simplify the situation (which could be an appropriate design choice anyway) is to make FullString a subtype of Object:

extension type FullString._(String value) implements Object {
  static FullString? fromOr(String value) {
    if (value.isEmpty) return null;
    return FullString._(value);
  }
}

This means that you're no longer encapsulating the representation type as much as previously, it is now guaranteed that the representation type is non-nullable. In return for that you get the ability to use a FullString in places where a non-null type is required.

However, this particular example is more tricky than that. ;-)

I can see that the binding of T in the invocation of the extension method is Object.

import 'dart:collection' as collection;

extension type FullString._(String value) {
  static FullString? fromOr(String value) {
    if (value.isEmpty) return null;
    return FullString._(value);
  }
}

// Copy `NullableIterableExtensions` from 'dart:core' in order to
// experiment with it. Change the member name to make sure this is
// the one we're calling.
extension NullableIterableExtensions<T extends Object> on Iterable<T?> {
  Iterable<T> get nonNulls2 {
    print(T); // Prints 'Object'.
    return collection.NullableIterableExtensions(this).nonNulls;
  }
}

void main() {
  var xs = ['Hi', '', 'Hello'].map(FullString.fromOr);
  xs.expectStaticType<Exactly<Iterable<FullString?>>>();
  xs.nonNulls2;
}

typedef Exactly<X> = X Function(X);
extension<X> on X { X expectStaticType<Y extends Exactly<X>>() => this; }

This program compiles and runs with no errors. This shows that xs has static type Iterable<FullString?>, and nonNulls2 is invoked such that T is bound to Object. In this case the instantiated on-type is Iterable<Object?>, which is OK because Iterable<FullString?> is a subtype of Iterable<Object?>.

[jellynoone]

Thank you for the explanation!

I was always under the impression an extension type is implicitly an Object. Since it isn't the reported error does make sense.

Please feel free to close this issue if everything is working as intend.

[eernstg]

@jellynoone, thanks for spotting this issue! The explanation was revised because it wasn't correct at first. But let's get this clarified. Nullability and extension types is a tricky topic. ;-)

[eernstg]

@chloestefantsova, @johnniwinther, @scheglov, sorry about the noise, this was a false alarm: The comment here has been corrected and updated, and it all seems to work correctly.

In particular, the extension on-type is instantiated as Iterable<Object?> because T is bound to Object, and this allows for a receiver of type Iterable<FullString?> (because that's a subtype of Iterable<Object?>).

[jellynoone]

@eernstg When I run your example with nonNulls2 with Dart SDK version: 3.8.1 (stable) (Wed May 28 00:47:25 2025 -0700) on "macos_arm64", I get the same error I got in the initial report (Error: The getter 'nonNulls2' isn't defined for the class 'Iterable<FullString?>'.).

[eernstg]

@jellynoone, that's the only missing part: I can't reproduce that error message using any version of the tool chain that I can think of.

It would be interesting to hear what the response is if you invoke the extension method explicitly and pass the type argument Object:

import 'dart:collection';

void main() {
  NullableIterableExtensions<Object>(['Hi', '', 'Hello'].map(FullString.fromOr)).nonNulls;
}

extension type FullString._(String value) {
  static FullString? fromOr(String value) {
    if (value.isEmpty) return null;
    return FullString._(value);
  }
}

[jellynoone]

@jellynoone, that's the only missing part: I can't reproduce that error message using any version of the tool chain that I can think of.

It would be interesting to hear what the response is if you invoke the extension method explicitly and pass the type argument Object:

import 'dart:collection';

void main() {
NullableIterableExtensions(['Hi', '', 'Hello'].map(FullString.fromOr)).nonNulls;
}

extension type FullString.(String value) {
static FullString? fromOr(String value) {
if (value.isEmpty) return null;
return FullString.(value);
}
}

@eernstg This works for me without error!

[jellynoone]

It is also strange to me that my editor offers autocompletion for nonNulls but then the analyser isn't happy.

[eernstg]

Interesting!

However, I can get the error with older versions of the tool chain, e.g., dart-sdk-3.6.0-122.0.dev. Perhaps you have more than one installation of the tool chain, and one of them is used on a command line whereas VScode or some other tool uses a different (older) one?

[jellynoone]

I'm on a MacBook Air M2, using Dart distributed through Homebrew which is Dart SDK version: 3.8.1 (stable) (Wed May 28 00:47:25 2025 -0700) on "macos_arm64".

I can reproduce the issue invoking the analysis via: /opt/homebrew/bin/dart analyze main.dart