Dart allows to extend deferred classes.

[iarkh]

Dart VM version: 2.9.0-10.0.dev (dev) (Tue May 19 15:16:48 2020 +0200) on "windows_x64"

The following source code declares a class which extends deferred class:

import "dart:core" deferred as core;

class A extends core.ArgumentError {}

main() {
  new A();
}

Analyzer throws a compile error here whereas dart passes without any errors and warnings.
Sample output is:

$> dartanalyzer test.dart
Analyzing test.dart...
error - Classes can't extend deferred classes. - test.dart:3:17 - extends_deferred_class
1 error found.

$> dart test.dart
$

Seems like according to the Dart Language Spec dart also should throw an exception here.

[johnniwinther]

See #30273

[sgrekhov]

The same issue exists for mixins as well. Analyzed doesn't allow to mixin deferred classes but CFE allows

import 'deferred_lib.dart' deferred as d;

class B {
}

class C extends B with d.A {} // No isses in CFE, in analyzer error - Classes can't mixin deferred classes.

main() {
  new C();
}

[sgrekhov]

The issue is still exists on CFE