Unhandled exception thrown if generic type is specified

[DartBot]

This issue was originally filed by andrejnav...@yahoo.com

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What steps will reproduce the problem?
If the following test program is executed then unhandled exception
is thrown. No exception is thrown if line (2) is put in comment,
and line (1) is uncommmented.

class _Iterable<E> implements Iterable<E> {
  final Iterator<E> _iterator;
  _Iterable(this._iterator);

  Iterator<E> iterator() {
    return _iterator;
  }
}

Iterable<Comparable> copy(Iterable<Comparable> iterable) {
  //return new _Iterable(iterable.iterator()); // (1) : OK

  // Unhandled exception: type '_Iterable@28152909<Comparable>' is not
  // a subtype of type 'Iterable<int>' of 'output'.
  return new _Iterable<Comparable>(iterable.iterator()); // (2) : FAIL
}

main() {
  Iterable<int> output = copy([0, 1]);
  Expect.isTrue(output.iterator().hasNext());
}

You may try the test program here http://try.dartlang.org/s/skw5

What is the expected output? What do you see instead?
Expected: no exception should be thrown, specifying generic type
should not influence on the execution of the program
Actual, Dartboard: Failed type check: type tator89da13$_Iterable is not assignable to type Iterable
Actual, Dart Editor: Unhandled exception:
type '_Iterable@28152909<Comparable>' is not a subtype of type 'Iterable<int>' of 'output'.
 0. Function: '::main' url: 'file:///C:/Dart/Projects/BugReport001/Test01/TestBugs.dart' line:18 col:30

What version of the product are you using? On what operating system?
Version 0.1.0.201204131908, Build 6557
Dart SDK version 6478
Windows 7 Home Premium, 64 bit

Please provide any additional information below.

[DartBot]

This comment was originally written by ladicek@gmail.com

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I still think that you should reach misc@dartlang.org instead of filing these issues. In this case, your problem isn't in the copy function

Iterable<Comparable> copy(Iterable<Comparable> iterable) {
return new _Iterable<Comparable>(iterable.iterator());
}

but in the main function instead:

Iterable<int> output = copy([0, 1]);

And only in checked mode -- if you disable type checks, your program runs just fine. Specifying types MUST influence program execution in checked mode, that's its very purpose!

Let's take a look at what happens:

1. You call 'copy' with a [0, 1], which is List, which is actually List<Dynamic> (if you'd like it to be List<int>, then you should write <int>[0, 1]). The 'copy' function expects Iterable<Comparable>, so it is OK (having Dynamic as type parameter is always OK in Dart, and passing List<int> would be just fine, because generics in Dart are covariant and int is a subtype of Comparable).
2. In 'copy', you first invoke 'iterable.iterator()', and since 'iterable' is Iterable<Comparable>, you get Iterator<Comparable>.
3. Then, you create _Iterable<Comparable>, which is OK -- it's constructor expects Iterator<Comparable> and you pass Iterator<Comparable> (you don't define a type in the constructor, actually -- as per the latest spec, the type is taken from the field in such situation, but this isn't currently implemented, AFAIK, so this check isn't currently performed).
4. You return the _Iterable<Comparable> as Iterable<Comparable>, which is OK.
5. Then, in 'main', you try to assign Iterable<Comparable> (returned from 'copy') to Iterable<int>, which fails. Remember -- you could assign Iterable<int> (or, in fact, List<int>) to Iterable<Comparable>, but you can't do it otherwise. Iterable<Comparable> isn't a subtype of Iterable<int> (generics are covariant, not contravariant).

[kasperl]

Let's take this discussion to the misc@dartlang.org mailing list. Feel free to re-open the issue if you feel like this really is a bug in Dart.

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Added Invalid label.