Compilation Error with Sendable

Using Swift
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1

Hi everyone.

I have a simple type alias: public typealias SendableCustomStringConvertible = Sendable & CustomStringConvertible

Trying to compile framework, I get a compiler error:
Undefined symbols for architecture x86_64:
"protocol descriptor for Swift.Sendable", referenced from:
type metadata accessor for Swift.CustomStringConvertible & Swift.Sendable in BaseError.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

The problem is that this error is not reproducable in another project. What are the possible reasons? What am I doing wrong?

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2

Is that typealias used anywhere in the project, or does the definition alone seem to break linking?

3

The alone definition leads to this error. One more interesting thing, the next lines of code don't lead to error:

extension OrderedDictionary: @unchecked Sendable where Key: Sendable, Value: Sendable {}
extension OrderedSet: @unchecked Sendable where Element: Sendable {}

extension NonEmpty: @unchecked Sendable where RawValue: Sendable {}

extension URL: @unchecked Sendable {}

The problem for now is only with typealis. But error occurs in another file. What I mean is typealias is defined in file A, and error occurs in file B.
If I replace typealias with direct protocol composition of Sendable & CustomStringConvertible, the error still occurs, only in file B and only inside this particular framework.

Some more details:

  • it is a dynamic framework
  • framework imports only one system library: Foundation
  • 3 libraries are added via SPM: SwiftCollections, NonEmpty, SwiftURL

The simplified code in file B is:

public protocol BaseError: LocalizedError, CustomStringConvertible, CustomDebugStringConvertible {
  var code: Int { get }
  
  var domain: String { get }
  
  var domainShortCode: String { get }
  
  var underlying: BaseError? { get }
  
  var userInfo: [String: any SendableCustomStringConvertible] { get }
}
4

Just while writing the message above I've got an idea: the minimal deploy target is iOS 12. So, I bumped it to iOS 13 and error has disappeared.
Thanks for response :pray:

5

Glad you figured it out! Still, I was under the impression that as a marker protocol, Sendable was meant to have no ABI impact, so I’m a bit confused why it would be showing up in linker errors… :thinking:

(And, regardless, it seems like availability checking should have thrown this out with a reasonable error messages as opposed to a linking failure)

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