Distributed: cannot use mutating member on immutable value, 'handler' is immutable

[MaxDesiatov]

Describe the bug
When trying to implement a type conforming to DistributedActorSystem an inscrutable error message is shown, which makes reference to identifiers that aren't present in the code. It's unclear how to fix this error. This is the only error shown by the compiler when using swift build or building in Xcode:

<unknown>:0: error: cannot use mutating member on immutable value: 'handler' is immutable

Complete build log can be seen here: https://github.com/MaxDesiatov/ClientServerRPC/runs/6659508563?check_suite_focus=true

To Reproduce
Steps to reproduce the behavior:

1. Clone https://github.com/MaxDesiatov/ClientServerRPC
2. Run swift build with latest 5.7 or trunk snapshot.

Expected behavior
Either the project builds or a clear actionable error message is presented.

Environment (please complete the following information):

• 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)

Additional context
Source code of the implementation is available at https://github.com/MaxDesiatov/ClientServerRPC/blob/6f50c1a/Sources/App/RPC/RPCSystem.swift

[ktoso]

Your handlers onReturn is mutating but should not be:

    <mutating> func onReturn<Success: SerializationRequirement>(value: Success) async throws {}

The ad-hoc requirement seems to miss checking for this; this is a missing error diagnosis that I'll fix, thanks 👍

(It is super unfortunate we have to do those ad-hoc but here we are...)

[ktoso]

rdar://94127604