Getting "unused" warning from function annotated with "@discardableResult"

[stephencelis]

Describe the bug

I've started to see unused warnings crop up in Xcode 14 beta 4 for endpoints that are annotated with @discardableResult.

Steps To Reproduce

It's not always easy to reproduce, but I've specifically gotten them to appear in the following repo/project/file:

Steps to reproduce the behavior:

1. Clone https://github.com/pointfreeco/swift-composable-architecture
2. Check out d526b07053d46a740936e50be436f78c7ce7b092
3. Open ComposableArchitecture.xcworkspace
4. Navigate to VoiceMemosTests.swift
5. Run tests, allow Xcode to type-check file, etc.
6. Note that lines like this one have the warning:

   Warning: Result of call to function returning 'TestStoreTask' is unused

7. Note that the function returning TestStoreTask is annotated with @discardableResult.

Expected behavior
I expect no warning.

Screenshots

Environment (please fill out the following information)

• OS: macOS 11.0
• Xcode 14 beta 4

Additional context
N/A

[tomguthrie]

We are also seeing this since beta 4. In our case it's only when the @discardableResult function is used within an async function / Task { ... }

[theblixguy]

Small self-contained reproducer:

@discardableResult @MainActor
func a() async -> Int { 5 }

func b() async {
  await a()
}

I get this on 5.7:

<source>:3:11: warning: result of call to function returning 'Int' is unused
    await a()
          ^~~

But no such warning on 5.6.

Also, if you remove the @MainActor annotation the warning goes away.

[theblixguy]

I have opened a PR to fix it.

[dominikmayer]

I'm still seeing the warnings in Xcode version 14.0 (14A309). @dfeinzimer mentioned the same in #57477. Do you want to reopen this issue or should I create a new one?

[youngToMaturity]

Same here with @dominikmayer. Xcode version 14.0 (14A309) and warnings only surface for async methods.

[dornad]

Same here with @loopbackseal. Xcode version 14.0.1 (14A400). Seems to still be contained to async methods.

[mredig]

It's happening to me regardless of async methods.

(and this is just one of a little over 100 similar warnings - unsure how many are async vs not)

(Xcode Version 14.0 (14A309))