[SR-11901] extensions on directly depended on types are polluted from transitive dependencies

[keith]

Previous ID	SR-11901
Radar	rdar://problem/16154294
Original Reporter	@keith
Type	Improvement

Attachment: Download

Additional Detail from JIRA

Votes	0
Component/s
Labels	Improvement
Assignee	None
Priority	Medium

md5: 1d53b710921ee26cf7a64bfe9dee53f4

Issue Description:

See the attached sample project and run `swift build` it should fail but it doesn't, see below for details.

Currently there is a case with transitive modules where you do not have to import a module to get extensions from it if you import the module containing the type its extending, and a module that depends on the module adding the extension. This is fixed if you add `@_implementationOnly` to the import of the module that adds the extension, but ideally it would be required to import any module you use an extension from (unless it's explicitly `@_exported`

In the sample project you can see that `import transitive` pollutes the `funcFromExtension` into the main `import-pollution` target. By changing the import to `@_implementationOnly import extender` you fix this issue.

I know this has been an issue for a long time since this also applies to extensions on types defined in system modules, and because of that it may never be "fixed" since it would be a source incompatible change, but I wanted to have an issue about this since I couldn't find any others.

In projects with many modules this makes it hard to keep your dependency graph actually explicit and introduces surprises when you remove a dependency on a module and end up losing transitive dependencies you were implicitly depending on.

[beccadax]

@swift-ci create

[tshortli]

Duplicate of #46493.