[bug] Pure pipes treated like impure pipes re. list values

[chalin]

Expected/desired behavior

See the Flying Heroes (pure pipe) example:

• Type in a hero name like "Birdman" and hit enter.

Since "can fly" and "mutate array" are selected, a new flying hero will be added to the existing hero list.

Since a pure pipe is used here, we should see "Birdman" appear in the All Heroes list, but NOT the Heroes who fly list.

This correct behavior was present up until beta.15. No statements can be made about beta.16 and beta.17 because pipes were broken (cf. angular/angular#8258).

Current behavior

"Birdman" appears in both lists, as if the pipe were impure.

Reproduction of the problem

Instructions are given above.

• Angular2 Dart version: beta.18
• Browser: tested on Chrome.

cc @kwalrath

[zoechi]

That's quite weird. Setting pure: true makes it work.
https://github.com/dart-lang/angular2/blob/61a4a504ea06cbad10f45563120a1d1dda4a890b/lib/src/core/metadata/directives.dart#L853
should return true in case there is no value passed, but somehow this doesn't seem to work.
Some contained pipes also seem to do that even though it shouldn't be necessary
https://github.com/dart-lang/angular2/search?utf8=%E2%9C%93&q=%22pure%3A+true%22

[matanlurey]

Just an update: Haven't had the chance to look at this 😢

This is great issue if anyone in the community is interested in contributing a fix and a test!

I have taken a quick look at the code around this bug today, and this change can fix it.
https://github.com/ntaoo/angular2/commit/1cafeabc58246b2b3f1a22929756aec250fc0e9c
(I confirm that with this fix, "Flying Heroes (pure pipe)" example works as expected.)

It looks like, at the compilation phase, _PipeMetadataVisitor visits pipe metadata AST and at the time PipeMetadata's constructor is not evaluated.

I'm not sure that it's the right way to fix it and don't write the test yet.
I'm reading the ng2 transformer with interest.