{"action":"opened","number":4155,"pull_request":{"url":"https://api.github.com/repos/apple/swift/pulls/4155","id":80666405,"html_url":"https://github.com/apple/swift/pull/4155","diff_url":"https://github.com/apple/swift/pull/4155.diff","patch_url":"https://github.com/apple/swift/pull/4155.patch","issue_url":"https://api.github.com/repos/apple/swift/issues/4155","number":4155,"state":"open","locked":false,"title":"[PrintAsObjC] Handle circularities introduced by ObjC generics.","user":{"login":"jrose-apple","id":15097531,"avatar_url":"https://avatars.githubusercontent.com/u/15097531?v=3","gravatar_id":"","url":"https://api.github.com/users/jrose-apple","html_url":"https://github.com/jrose-apple","followers_url":"https://api.github.com/users/jrose-apple/followers","following_url":"https://api.github.com/users/jrose-apple/following{/other_user}","gists_url":"https://api.github.com/users/jrose-apple/gists{/gist_id}","starred_url":"https://api.github.com/users/jrose-apple/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/jrose-apple/subscriptions","organizations_url":"https://api.github.com/users/jrose-apple/orgs","repos_url":"https://api.github.com/users/jrose-apple/repos","events_url":"https://api.github.com/users/jrose-apple/events{/privacy}","received_events_url":"https://api.github.com/users/jrose-apple/received_events","type":"User","site_admin":false},"body":"Like Swift generics, Objective-C generics may have constraints; unlike Swift generics, Objective-C doesn't do separate parsing and type-checking passes. This means that any generic arguments for constrained generic parameters must be fully-defined, in order to check that they satisfy the constraints.\r\n\r\nThis commit addresses this problem with three different mechanisms, one for each kind of declaration that might run into this issue:\r\n\r\n- For classes, if a member references a type with constrained generic parameter, and the corresponding argument type hasn't been printed yet, that member is \"delayed\", which means it is put into a category at the end of the file.\r\n\r\n- Protocols cannot have categories, so for protocols we instead see if we can print the definition of the other type first. To break circular dependencies, the printer will not attempt this if both the type and the protocol are already being depended on. This isn't perfect (see below).\r\n\r\n- Rather than delaying members of extensions, we just delay them wholesale. This keeps related members together, but also has problems (see below).\r\n\r\nThese approaches solve the most common cases while still not crashing in the uncommon ones. However, there are still a number of problems:\r\n\r\n- The protocol heuristic is overly negative, which means we may generate an invalid header even when there's a reasonable ordering. For example, a single class might inherit from a class A and conform to protocol P, and protocol P depends on class A as a generic argument. In this case, defining class A first is the right thing to do, but it's possible for the printer to decide that there's circularity here and just forward-declare A instead.\r\n\r\n- Protocols really can be circular. This can be fixed by printing a forward-declared protocol alongside the generic constraints, i.e. `id <MoreThanNSCopying, NSCopying>` instead of just `id <MoreThanNSCopying>`.\r\n\r\n- Extensions can introduce protocols as well. This is not modeled at all; if a member depends on a protocol conformance, it's assumed that simply printing the class would be sufficient. This could be fixed by checking how a generic argument satisfies its constraints, possibly delaying individual members from extensions in order to print them sooner.\r\n\r\n- More cases I haven't thought about.\r\n\r\nTest cases for some of these problems are in the new circularity-errors.swift file, mostly to make sure the ObjC printer doesn't crash when it encounters them. I intend to file bugs for these issues.\r\n\r\nrdar://problem/27109377\r\n\r\n* * * *\r\n\r\n<!-- This selection should only be completed by Swift admin -->\r\nBefore merging this pull request to apple/swift repository:\r\n- [ ] Test pull request on Swift continuous integration.\r\n\r\n<details>\r\n  <summary>Triggering Swift CI</summary>\r\n\r\nThe swift-ci is triggered by writing a comment on this PR addressed to the GitHub user @swift-ci. Different tests will run depending on the specific comment that you use. The currently available comments are:\r\n\r\n**Smoke Testing**\r\n\r\n        Platform     | Comment\r\n        ------------ | -------------\r\n        All supported platforms     | @swift-ci Please smoke test\r\n        All supported platforms     | @swift-ci Please smoke test and merge\r\n        OS X platform               | @swift-ci Please smoke test OS X platform\r\n        Linux platform              | @swift-ci Please smoke test Linux platform\r\n\r\nA smoke test on macOS does the following:\r\n\r\n1. Builds the compiler incrementally.\r\n2. Builds the standard library only for macOS. Simulator standard libraries and\r\n   device standard libraries are not built.\r\n3. lldb is not built.\r\n4. The test and validation-test targets are run only for macOS. The optimized\r\n   version of these tests are not run.\r\n\r\nA smoke test on Linux does the following:\r\n\r\n1. Builds the compiler incrementally.\r\n2. Builds the standard library incrementally.\r\n3. lldb is built incrementally.\r\n4. The swift test and validation-test targets are run. The optimized version of these\r\n   tests are not run.\r\n5. lldb is tested.\r\n\r\n**Validation Testing**\r\n\r\n        Platform     | Comment\r\n        ------------ | -------------\r\n        All supported platforms     | @swift-ci Please test\r\n        All supported platforms     | @swift-ci Please test and merge\r\n        OS X platform               | @swift-ci Please test OS X platform\r\n        OS X platform               | @swift-ci Please benchmark\r\n        Linux platform              | @swift-ci Please test Linux platform\r\n\r\n\r\n**Lint Testing**\r\n\r\n        Language     | Comment\r\n        ------------ | -------------\r\n        Python       | @swift-ci Please Python lint\r\n\r\nNote: Only members of the Apple organization can trigger swift-ci.\r\n</details>\r\n<!-- Thank you for your contribution to Swift! 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