Distributed: incorrect diagnostics for missing import

[MaxDesiatov]

Describe the bug
When import Distributed is missing in a file which has distributed actor with distributed func in it, an seemingly unrelated error messages are produced:

error: type 'Greeter' does not conform to protocol 'Equatable'
...
<unknown>:0: error: cannot find '__isRemoteActor' in scope
<unknown>:0: error: cannot find 'RemoteCallTarget' in scope

To Reproduce

1. Create a file with this content (assuming an implementation YourClusterSystem of DistributedActorSystem is in scope)

typealias DefaultDistributedActorSystem = YourClusterSystem

distributed actor Greeter {
    distributed func hello() {
        print("Hello, Distributed World!")
    }
}

2. Try to build and observe unrelated error messages that don't mention the missing import.
3. Adding import Distributed in such file makes the errors go away.

Expected behavior
If import Distributed is always needed in files that contain distributed actors, an error message that requires the import should be displayed. Unrelated diagnostics about missing Equatable conformance shouldn't be displayed as it's confusing.

Environment:
OS: macOS 12.4 (21F79), but most likely reproducible on any platform that supports distributed actors.
Xcode Version/Tag/Branch: 13.4 (13F17a)
Toolchain: Swift Development Snapshot 2022-05-27, also reproducible with 5.7 Development Snapshot 2022-05-18 (a)

[ktoso]

Thanks, I’ve known about that one but didn’t seem to have a public bug about it — thanks for reporting!

yes, the diagnosis should be about the missing import indeed :-)

[ktoso]

Note to self, also add a test with an extension in separate module that forgot to do the import

[akbashev]

Issue is still there, just saying.