Dart fails with class A<X extends FutureOr<A<X>>>

[iarkh]

Dart SDK Version: 2.1.0-dev.8.0
OS: Windows 10 (64 bit)

This is a regression in dev.8.0, dart passes with the same code sample with dev.7.1.

Please note that there is an opened bug #34264 regarding dartanalyzer.

Here is a sample code example:

import "dart:async";
class A<X extends FutureOr<A<X>>> {}
main() {}

Still analyzer fails here because of #34264.

But in the recent dart version dart started to fail here too, sample output is:

test.dart:2:9: Error: Type argument '#lib1::A::X' violates the corresponding type variable bound of 'A'.
Try changing type arguments so that they conform to the bounds.
class A<X extends FutureOr<A>> {}
^
test.dart:2:9: Context: Bound of this variable is violated.
class A<X extends FutureOr<A>> {}
^
test.dart:1: Error: Type argument '#lib1::A::X' violates the corresponding type variable bound of 'A' in the return type.
Try changing type arguments so that they conform to the bounds.
test.dart:2:9: Context: Bound of this variable is violated.
class A<X extends FutureOr<A>> {}
^

[iarkh]

Seems like the following code fails with dart because of the same reason:

import "dart:async";
class A<X extends A<X>> {}
class B<X extends A<A<X>>> {}
main() {}

Sample output is:

test.dart:3:9: Error: Type argument '#lib1::A<#lib1::B::X>' violates the corresponding type variable bound of 'A'.
Try changing type arguments so that they conform to the bounds.
class B<X extends A<A>> {}
^
test.dart:2:9: Context: Bound of this variable is violated.
class A<X extends A> {}
^
test.dart:3:9: Error: Type argument '#lib1::B::X' violates the corresponding type variable bound of 'A'.
Try changing type arguments so that they conform to the bounds.
class B<X extends A<A>> {}
^
test.dart:2:9: Context: Bound of this variable is violated.
class A<X extends A> {}
^

So please let me know if I should file a separate issue about this.

[vsmenon]

Filing on language to clarify behavior and/or whether this is an intended breaking change.

[iarkh]

This bug is still reproducible with the recent dart version:
Dart SDK version: 2.11.0-176.0.dev (dev) (Mon Sep 28 19:05:52 2020 -0700) on "windows_x64"

[scheglov]

The fix landed as 499bae0.

It seems that this change should be in 2.11.0-176.0.dev. When I try with the bleeding edge build, it seems to work.

scheglov@scheglov-macbookpro2:~/Source/Dart/sdk.git/sdk (master)$ cat /Users/scheglov/dart/test/bin/test.dart
import "dart:async";

class B<X extends Future<A<X>>> {}
class A<X extends FutureOr<A<X>>> {}

A a = 0;
B b = 0;
scheglov@scheglov-macbookpro2:~/Source/Dart/sdk.git/sdk (master)$ dartanalyzer --version
dartanalyzer version 2.11.0-edge.919f68d8ac2da1f1498bceb3cda610b4b0ce9905
scheglov@scheglov-macbookpro2:~/Source/Dart/sdk.git/sdk (master)$ dartanalyzer /Users/scheglov/dart/test/bin/test.dart
Analyzing /Users/scheglov/dart/test/bin/test.dart...
  error • A value of type 'int' can't be assigned to a variable of type 'A<FutureOr<A<dynamic>>>'. • /Users/scheglov/dart/test/bin/test.dart:6:7 • invalid_assignment
  error • A value of type 'int' can't be assigned to a variable of type 'B<Future<A<dynamic>>>'. • /Users/scheglov/dart/test/bin/test.dart:7:7 • invalid_assignment
2 errors found.

[eernstg]

Closing: The example program is accepted by dart and dartanalyzer as of a49217b.