1

I was updating some tests and noticed this strange warning.

/Users/varun/foss-swift/swift/test/ModuleInterface/access-filter.swift:130:8: warning: instance method 'requirement()' should be declared '@usableFromInline' because it matches a requirement in protocol 'PublicProto'
  func requirement() {}
       ^

The test also has a FIXME:

  // FIXME: Is it okay for this non-@usableFromInline implementation to satisfy
  // the protocol?

Why would this not be okay? Should we just remove the FIXME? I imagine we can't change this to an error anyways as that would break source compat.

2

Presumably the sense in which it would not be ok is that there is nothing to indicate that this method is a part of the ABI of the module, but it appears to be being used as one.

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3

Basically yeah. Normally adding a protocol conformance can’t expose any private or internal members as API, but there was a hole in access checking around protocol extension members. I don’t actually remember the specifics for @usableFromInline but I assume it’s that once you accept the former breakage, you now question whether the ABI-ness should be marked in some way.

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