dart:mirrors does not yet work with --preview-dart-2

[itsjoeconway]

This code prints false when running with --preview-dart-2.

import 'dart:mirrors';

enum Enum {
  option1, option2
}

void main() {
  final isEnum = reflectClass(Enum).isEnum;
  print("${isEnum}");
}

[kmillikin]

Support for dart:mirrors is currently unimplemented with --preview-dart-2. Some things will happen to work, but nothing is guaranteed to work. The things that happen to work today might randomly break tomorrow.

As far as I know, we have not made any official decision about supporting dart:mirrors on the Dart VM. It is not supported one the Web in Dart 2.0 (#30538) and it's also not supported in the Flutter mobile development framework (flutter/flutter#1150).

[kmillikin]

And now I do know: we have made an official announcement that dart:mirrors will continue to be fully supported on the Dart VM in Dart 2.0. It is here: https://groups.google.com/a/dartlang.org/d/msg/misc/djfFMNCWmkE/F7WE8a0JAwAJ

[itsjoeconway]

Thanks very much for the info, will continue to track issue

[a-siva]

The root cause of the enum not being report as 'isEnum' might be the same one as in #32365

[bergwerf]

Phew, I'm glad dart:mirrors will continue to be supported, without it my code generation would become a bit cumbersome...
Also walked into the isEnum bug, but I see it is already fixed in the other thread (so will have to wait for a new dev release now)

[manuelF]

Also running into issues with preview Dart 2. Package unscripted [0] requires dart mirrors fetching method parameters and it fails with a runtime type issue:

type 'List' is not a subtype of type 'List' of 'function result' where
List is from dart:core
List is from dart:core
ParameterMirror is from dart:mirrors

dart:mirrors _LocalMethodMirror.parameters

[0] https://github.com/seaneagan/unscripted/blob/master/lib/src/util.dart#L38

[dgrove]

@vsmenon do you have any needs here?

[vsmenon]

@jmesserly

Not that we're aware of at this point. The CFE recently added support for parameter annotations (which we needed for temporary pageloader support).

[jmesserly]

As far as I know, mirrors has enough support for DDC+Kernel. We have limited test coverage though, outside of the internal use of pageloader. (When I've regressed something in DDC+Analyzer mirrors, I don't find out until we roll the SDK.)

[a-siva]

It appears that there are a number of VM mirrors tests that are marked as failing in the status files which indicates we still have a number of issues with dart:mirrors functionality in --preview-dart-2 mode.

[kmillikin]

@manuelF Package unscripted itself is not a Dart 2 package. I run into a number of Dart 2 static errors when I try to compile it. (That's not to say that all the dart:mirrors stuff works there, but just that I haven't tested it).

[kmillikin]

I started to try to fix the failing mirrors tests. At least some of them are incorrect tests. We could argue about them on a case-by-case basis, because the current VM behavior is "incorrect" by that standard and changing the VM's behavior is a breaking change.

For example, in Dart 2 the classes defined by anonymous mixin applications should always be abstract, otherwise you will get Dart 2 static errors for any abstract members of the mixed-in class. One of the lib_2/mirrors tests tests that they are not abstract.

[kmillikin]

Another example: if an object that is not a Function is passed as the first argument to Function.apply, we will tear off the object's call method and pass that. If it is invoked with an incorrect number of arguments, that will not invoke the original object's noSuchMethod. lib_2/mirrors/apply3_test is broken for this reason.

So far, every test I've looked at is a test that was incorrectly ported from Dart 1. We'll need to fix them eventually but it isn't a high priority right now.

[jmesserly]

So far, every test I've looked at is a test that was incorrectly ported from Dart 1. We'll need to fix them eventually but it isn't a high priority right now.

yeah, I saw that when I moved these tests to lib_2/mirrors :\ ... a lot of tests in lib_2/corelib_2/language_2 still don't expect correct runtime behavior for Dart 2. I think the function call tearoff thing happened after the Dart 2 test migration, though. My guess is: we didn't update the tests after the call tearoff change, because many Dart 2 impls lack mirrors support, so it was hard to notice the test problem.

[itsjoeconway]

If there is any way to contribute here as an outsider, please let me know. The issues with mirrors and --preview-dart-2 flag are blocking many of Aqueduct tests. I am anxious that this issue may get deferred until after Dart 2 release or late enough in the process that will delay us releasing with Dart 2. I recognize that there are many other priorities and y'all are killing it, but I did want to express that concern.

[dgrove]

We're definitely aware of this concern. A prioritized list of APIs that you need would be very helpful.

…

On Tue, May 22, 2018 at 12:42 PM Joe Conway ***@***.***> wrote: If there is any way to contribute here as an outsider, please let me know. The issues with mirrors and --preview-dart-2 flag are blocking many of Aqueduct tests. I am anxious that this issue may get deferred until after Dart 2 release or late enough in the process that will delay us releasing with Dart 2. I recognize that there are many other priorities and y'all are killing it, but I did want to express that concern. — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#32345 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ACAsW76fx9eLSaMWNJF-EcKG2UwXQwMQks5t1GoPgaJpZM4SV7lE> .

[kmillikin]

To echo what @dgrove said, we could definitely use help identifying the high priority issues that affect you and your tests. You can file separate issues for them and just tag me for triage.

Eventually we will fix all those bugs (and probably modernize the API for Dart 2 features), but for now we want to prioritize fixing bugs "on demand" when we know that they are affecting customers.

[itsjoeconway]

Very much appreciated. I've added #33207 and will tag you shortly.

[natebosch]

@kmillikin - can we close this? I think mirrors are working now in the VM?

[a-siva]

all reported issues against dart:mirrors has been fixed after the switch to Dart 2.