Sendable warning produced for unused non-Sendable result value

[itaiferber]

(Migrating details from the Swift Forums)

Description

When Targeted/Complete Sendable checking is enabled, the compiler produces a warning for non-Sendable types crossing actor boundaries as result values, even if the values are explicitly unused.

Steps to reproduce

Compile the following code with Swift 5.8:

@globalActor
final class SomeDedicatedActor {
    actor Actor {}
    static let shared = Actor()
}

@SomeDedicatedActor
func f() async -> [Any] {
    // ...
}

@MainActor
func g() async {
    // ⚠️ Non-sendable type '[Any]' returned by implicitly asynchronous call
    // to global actor 'SomeDedicatedActor'-isolated function cannot cross
    // actor boundary
    _ = await f()
}

Expected behavior

I would expect Swift to

1. Attempt to elide transferring the resulting value across actor boundaries, and release it on the original actor
2. Avoid producing the diagnostic

Environment

• Swift version:

  swift-driver version: 1.75.2 Apple Swift version 5.8 (swiftlang-5.8.0.124.2 clang-1403.0.22.11.100)
  Target: arm64-apple-macosx13.0
  

• Xcode version:

  Xcode 14.3
  Build version 14E222b