Implementation issue for superinterface contravariance error Q1'19

[eernstg]

This issue is for tracking implementation of the error raised for non-covariant occurrences of a type variable in a superinterface of a generic class:

Request issue: #38
Feature issue: #39

Feature specification:
https://github.com/dart-lang/language/blob/master/accepted/future-releases/contravariant-superinterface-2018/feature-specification.md

Implementation plan:
https://github.com/dart-lang/language/blob/master/accepted/future-releases/contravariant-superinterface-2018/implementation-plan.md

ETA: Q1 2019 in Dart Repo.

Breaking change request: dart-lang/sdk#35097

Phase 0:

• Implementation in Analyzer (Analyzer: Enforce covariance of type variables in super-interfaces sdk#35553)
• Implementation in CFE/Kernel (Implement contravariant-superinterface-2018 in Fasta sdk#35792)
• Implementation in Dart2js (TODO)
• Implementation in DDC (TODO)
• Write language tests (Write language tests for covariant-only superinterfaces sdk#36231)
• Implementation in Intellij (TODO)

Phase 1:

• Language Specification Update (Specify superinterface contravariance error #387)
• Issue for updating documentation (TODO)
• Announcement (TODO)

[leafpetersen]

@kmillikin @stereotype441 Any updates on schedule? Still on track for Q1? Is there a tracking issue for the CFE?

[kmillikin]

There's no tracking issue for the CFE. It's still on track for Q1 in the sense that our estimate is that it's a week's worth of work. It's not on track for Q1 in the sense that we haven't started it so we're 0% completed at this time.

[kmillikin]

Filed dart-lang/sdk#35792

[stereotype441]

The analyzer is in a similar situation - we haven't started, but it should take no more than a week's worth of work, and we expect to be able to get to it this quarter.

[leafpetersen]

@eernstg Are there any language tests for this?

[eernstg]

I do not think we have any language tests for this, I'll write some. Changed the initial text of this issue to include a link to the issue.

[eernstg]

Language tests for this landed as dart-lang/sdk@2805a37.

[leafpetersen]

cc @kevmoo

This has landed in bleeding edge, for both CFE and analyzer.