一応の補足。プロポーザルに書いてあった。Error: Sendable

A function or closure that throws can effectively return a value of any type that conforms to the Error protocol. If the function is called from a different concurrency domain, the thrown value can be passed across it. To close this safety hole, we alter the definition of the Error protocol to require that all error types conform to Sendable:

https://github.com/apple/swift-evolution/blob/main/proposals/0302-concurrent-value-and-concurrent-closures.md#thrown-errors

swift-evolution/0302-concurrent-value-and-concurrent-closures.md at...

This maintains proposals for changes and user-visible enhancements to the Swift Programming Language. - swift-evolution/0302-concurrent-value-and-concurrent-closures.md at main · apple/swift-evolution